I don't fish the Thames tony, but I have a few boats and mainly use them on the Severn.
You really need to decide what type of fishing you are going to do first....
I mainly predator fish from my boat, but I am planning on doing some barbel fishing from it next season.
If you are going to fish for barbel (or carp), doing nights, then something with a cuddy as john says, will be ideal.
If you are going to lure fish for pike/perch/zander, then an open top boat with a pram hood cuddy will probably be better.
Obviously, ally boats are lighter, but more expensive, fibreglass are heavy, but I've found them more user friendly.
You also need to think about storage, or are you planning on keeping it in a marina.
Also, will it just be you, or will you have a boat partner.... Launching and retrieving on your one can be challenging at times.
Regarding engines, I run a 10hp mariner, but I've played about with several different engines, trying to do it on the cheap... Don't bother.
Get yourself a decent outboard from the start and look after it....
An electric outboard is also useful, especially for sneaking into spots quietly.....
Steve