Hi - I visited Exeter School, St Margarets and Maynard (girls only) when I first arrived here and had daughters in different primaries. I like Exeter School Junior School, but don't think it would suits my DDs as it seems to be VERY sporty (although I believe music and drama are strong too, especially in the senior school) and still a bit of a boys school IYSWIM, even though it is co-ed. I was also put off by the children who showed me round who were lovely, confident etc but clearly EXTREMELY wealthy (lots of talk of their ponies and swimming pools). Lots of flashy cars outside too... we're not poor, but not quite in their league! St Margarets is more homey and less of a big formal school, but the class sizes are so small in the Junior School that they combine classes which put me off. In the end, I decided that our (excellent) village primary was a better bet for us (and DD1 got a place there after a year). I will review at Yr 6, and on the basis of my visit, would consider all three (I figure there will be more of a social mix in the seniors at ES).
Didn't visit ECS, but it is also boarding, which throws the boarding/day-pupil dynamics in, which I wanted to avoid as DH hated this at his school. (I realise I am starting to sound VERY fussy!).
As for buses, I know that Exeter School for one, do buses - they seem to be pretty much organised by the parent community, but they had a list of the buses at their open day. If you email them, I am sure you can get one. From memory, they were from quite far away - we (20 mins from Exeter in East Devon) were way too near!
Finally, watch out for the trains - the stations are not necessarily near the schools. St Davids is on the opposite side of the city from St M's and ES; Central Station might work for the ECS.
No idea about bursaries; I assume they have some, but you'd best enquire with the school. I found ES and St M's very approachable and once you are on their mailing list there is quite a lot of communication!!
Good luck.