Pmart - I have been to East Africa twice and have used Intrepid/Guerba on both occasions. The safari you are on is one that tends to crame as many 'high-lights' as possible into the 14 days (btw - the first night in the Pan-Africa hotel is counted as day 1). Given the state of the roads in Kenya I recommend that you bring a small cushion/pillow to try to soften the pot-holes. The 4-wheel drive van that we used last Aug 2007 are being phased out in favour of larger Toyotas and Land-Rovers though I do not know how far advanced the program is. The vans are a better bet if there are only 6 (max 8) trekkers. You are going to have some long-long days, particularly Kisii to Lake Victoria and NGO crater (which should now be a half-day game drive) and then to Mesrani - which is just across the Kenya/TZ border. I suggest that you will be very tired when you arrive back in NBO. The above said - the drivers are usually very good at their jobs, they know the country and speak reasonable english. The camp boss (the cook) will also have reasonable english although the camp crew may have a limited vocab (as long as you are friendly towards them a smile says a lot more than words), Given that you are crossing the border you may have two different camp crews - on for Kenya and another for TZ. Try to check this one out - you may be up to two lots of tips for the camp crews, although you should have the one driver/guide for the whole safari.
Intrepid is an Australian company you bought out or merged with Guerba about 2 years ago. The guide will double check that you have the necessary insurance, I recommend you enquire about one that includes repatriation back to your home country if necessary. (sorry if this is a bit long)
Best wishes for a great trip. Regards Orion_Mike