Before you go spending mad, I’d do some local research on rights of way/local clubs/facilities, I gave up trail riding (and TRF) quite a while ago as so many local lanes got closed down/reclassified that there just aren’t enough local places to ride to make it worthwhile. I ended up doing holidays abroad (Greece, Crete, Spain etc as they are less restrictive). Try the TRF website and maybe attend a local meet. http://www.trf.org.uk/, see also the TBM magazine which is in some shops for ads etc. http://www.trailbikemag.com/
Offroad motorbiking (IMHO) can be more physically demanding than mtb (esp in deep mud and dragging) but to be fair some of the best fun I ever had was trail riding.
There are a few places that you can hire an MX at a track, well worth it and it will give you a feel for it. (Yamaha and Honda used to run sessions) http://www.yamaha-offroad-experience.co.uk/ as above
Re bike, if you only want offroad/trail riding enduro bike probably better, if you want do some road as well then trail may be better.
The xr honda range is bombproof and common as muck (but does the job), if you do buy an enduro (as opposed to trail) bike then check things like front/rear brake lights, horn, numberplate light, speedo etc for full legality as some plod and mot inspectors can be over zealous.
I’m a bit out of date but as a trailie DR350 (probably hard to get a good one now) used to suit larger lads, who did real offroad, the lighter the better is a reasonable rule and fit wrapround handguards (bike spares are expensive..)
Overall it’s definitely one of the best parts of my life, so so so many fabulous memories, a fantastic way to get out and about, sadly now no longer in favour….good luck