What bike depends on how they have been looked after, all are good and dont forget Scorpa off your list. My least favourite are Sherco mate had one and it was horrid, Gas Gas are ok but build quality is a bit hit and miss. Had a Beta which was ace, the stator went which they all do 😆 also had a couple of Scorpas and they are bullet proof, Yamaha engine but they went bust, parts are no problem Birket trial sport were the importers.
Who is you local dealer and what do they sell?
Montessa lovely but getting on a bit unless you go 4 stroke which you wont get for £1500.
Twin shock is something to think about.
Trials is ace and the only person you are competing with is yourself. Practice practice practice as much as you can, dont enter a Center event first time as you will just give up, find a club that has proper beginner sections and have fun fun fun