What I meant was Landmannaugar to Thorsmork then back to Landmannaugar again
Is that really how you want to spend your holiday time? If you take bus to L, walk in 3 days, and bus out of T, then at the reduced frequencies of bus services during September probably that takes 4 days in all, though given you were planning to walk it twice in 5 days, maybe you can get into T before the bus leaves and get in and out in just 3 days (remembering that you won' t be leaving L until lunchtime on the first day, and it is very scenic around L so it is tricky to march out at full walking speed with so many interesting things to look at, and the desire to do that warm swim in the stream).
Is there anything that I need to watch out during the end of the season?
The later you get, the greater the risk of snow. There can be light snow at Landmannalaugar itself (600m) in early Sept, and when you go up the hill (to as high as about 1100m on the trail) there can be a lot of snow. You can also be lucky and have nice weather...
Car rental cost 57 euros/day for their smallest 4 x 4
xrates.com says the x-rate is currently 148/Euro, so that is about 8400 K/day, which is cheaper than the bus for 4 days but more expensive than the bus for 5 or 6 days.
At the end of the day, you can do the trek, walk it just one way, and it costs you the bus fares. You can also choose to spend some extra time in L or T, which are nice places to visit, if the weather isn't horrible (a big if), or walk on from T to Skogar, which is exceedingly worthwhile, and it won't cost you any more, even if you have spent 6-7 days over it all.