Some people like C150's and others (myself included don't).
Personally as an instructor who has given hundreds of trial flights its worth the extra to have a 4 seater and to take another 2 people along to share the experence. It gives another 2 people a share of the views but also give quite a bit of scope for banter. They can also take photo's of the trip.
You do need a med cert but it all depends on which license you want to go for.
If you are going for a day only flying in the UK licsense (NPPL) your doctor can do it.
If you want the international license which you can add to you need to go to an approved medic. (PPL)
If you are only learning to fly as a hobby you need to do a fair bit of reading to work out what you actually want to fly. The PPL will cover you for most things with very little hassle but is more expensive than some of the other options which are more restrictive.
Now not implying anything about the schools that have already been mentioned but a recuring theme with flying schools is they have a habit of going bust. Please don't pay anything up front for a discount unless you can afford to loose the lot. Cessna schools (ie ones that use cessna as there training aircraft) I would be very wary of because there has been some compulsory engineering work been issued for most cessna types which is very expensive.
Other aircraft types you need to have a look at are microlights weight shift where you sit in a bath under scaffolding polls and canvas (but great fun apparently and very cheap in the grand scale of things)
3 axis microlights which are diddy fixed wing aircraft but can out perform the lower spec fixed wing aircraft.
To be honest sitting down with a "new" student interested in flying going through all the options would take an hour with a whiteboard.