Aconcagua guidance

Many people start with another 5000 meter trek before attempting Aconcagua, like the Cordon del Plata in Vallecitos, or El Plomo above Santiago;

If you had no problem with altitude on EBC/KP then you should have no problem on Aconcagua with proper acclimatisation. Your lack of experience is not a problem if you go with a professional and qualified guide. The ordinary route is technically easy. The real problems are staying healthy, physical exertion, and weather. You will probably need to be fit enough to carry a substantial load to at least one higher camp.

For further information see, for example: http://summitclimb.com/new/default.asp?vid=90

I have not climbed Aconcagua, but from the numerous reports it seems the main problems are insufficient acclimatization (itinerary too fast), stomach problems and weather. The first two are preventable (but lingering in the unhygienic high camps to avoid AMS adds the risk of the other). Many operators are not really interested in the success of the clients but getting their business = money. The climb itself is not technically demanding.

Actually, there are no guarantees with altitude. There is no correlation between the risk of altitude sickness and previous experience at altitude or fitness. You can be fine on one trip at altitude and have horrible altitude sickness on your next trip. So prepare well, but accept that there's always the chance you won't make it to the top.

Yes, operators are definitely interested in $$. For any trek or mountaineering trip, your best bets are to do your research well. This is not a time to go for the cheapest option - choose a company that allows enough time for acclimatization and will have at least one IFMGA certified guide on the trip. The latter ensures a certain standard of training and experience, as well as standards as to insurance carried and codes of practice (i.e. limits on situations/difficulties to which they will take clients). The main route is not really technical, but you want a qualified guide making decisions, especially if something happens.

Quote from #4: "There is no correlation between the risk of altitude sickness and previous experience at altitude or fitness. You can be fine on one trip at altitude and have horrible altitude sickness on your next trip. "

To a certain extent this is true, but on the other hand the acclimatization ability is a genetical trait, some people adapt better than average every time, some people have always problems or are even unable to climb higher than about 4300m or so. If somebody has done a number of trips to high altitudes and has adapted every time better than average, it is highly likely that he/she will adapt better than average also the next time (if his/her general health has stayed the same). I have been over 20 times at 5000+ meters, twice over 6000m, and I have never scored higher than 2 points on the Lake Louise score card, while a number of people in the same party have suffered AMS even to evacuation situation level, or have had to turn back under their own power. I am quite confident I acclimate considerably better than average and do not worry about altitude. Knowing your acclimatization ability requires testing it, either by climbing high repeatedly and getting to know this by trial and error, or by having expensive DNA tests done to find if one has certain genes identified with good acclimatization ability, there are about 20 of those known at the moment. Over 90% of successful 8000m climbers have those genes, no surprise there.

I can offer some advice on both Aconcagua and the Inca Trail having done both (attempt only on Aconcagua).

The Inca Trail (the most popular one as there are other "Inca Trails") is booked out months in advance. You have to go with a trekking company/ get permit etc. I went in 2006 and the wait then was several months. I understand it is now even longer so you may not be able to do the trek when you want to.
One other post recommended the Huayuash Circuit or the Ausangate Circuit as a warm up to Aconcagua. I did the Ausangate circuit , also in 2006. However I followed on from the Inca Trail (June) and organised everything myself while in Cusco. As I was already acclimatised I found it good going (several passes around 5000 metres). As I recall I paid about $US 450 at the time for the following :- a great English speaking guide, two horses, two horsemen, a cook, taxi from Cusco to the trail head and back (quite a distance on a rough road but I believe the road is a lot better now). I had a wonderful trip with these people and the scenery was amazing . The trekking season is April to October by the way.
With regards Aconcagua - I went with an Australia based company (they also do climbing expeditions to K2, Everest etc). Of about 12 trekkers/climbers, only 4 summited due to very bad weather conditions on the day that most of us attempted the summit (4 retired at various heights due to altitude). I reached a personal best of 6400 metres in minus 40 windchill (January) - crampons and ice axe were required as it was a very snowy/icy season (2008-09). Prior to our expedition around 5-6 persons died on Aconcagua in the week before we started. So yes, it can be a very serious undertaking. Don't be fooled by anyone who says it is a "walk up". Even in the rarer benign weather days, it is very physically demanding and altitude sickness is a very real eventuality. One of those who died (as mentioned) was a "very fit young man" who was almost at the summit (he dropped dead on the spot) . By the way, the other deaths were from stonefall (one) and 3 died on a more difficult route. Even when we had to break camp at 6000 metres I ended up with frost-bite caused by having to remove the bulky gloves untying the tent guy-ropes. You really need to go with a certified company, local or international. Doctors at the two main base camps won't let you go higher if you don't pass certain medical checks. I've been over 6000 metres on at least 5 occasions (different trips) and haven't had a problem with altitude but I think 7000 metres would be my upper limit.
I prefer not to use Diamox by the way and if used, half a tab seemed all I needed. I'd rather take longer and acclimatise as natural as possible.
As a postscript: I did another trip with the Aconcagua outfit I was with - I went as a trekker to K2 base camp and over the Gondogoro La (almost 6000 metres) in June 2009. An amazing trip - in my top 3. Truly spectacular scenery. You would need to go with a good trekking company as there is the on-going security problem in Pakistan.
I hope your plans work out. Sound preparation will almost guarantee a great trip. I've now walked/climbed in 34 different countries and never had a dud yet. You can do so much planning on-line and there are so many good operators now you shouldn't go wrong. Budget trips aren't necessarily the best for quality but having said that in some places you get really amazing value for your dollar. Don't forget to make a realistic tip though as you're putting money directly into the pockets of the locals who can benefit so much from what you give them.