Hey welcome to trials man 26" would be the best option to go for based on your riding background but, you have to bear in mind the radical geometries of pure trials frames if you're dead set on one. The cockpit will feel more compact but higher than what you're used to so the higher BB frames (e.g. +70mm) will feel really odd to ride coming off a MTB background - as they're better suited doing moves on one wheel than two. But for any rolling moves like manuals and bunnies, it will feel shit.
Have you considered the Inspired Hex? The geometry is much better balanced than pure trials frames, allowing you to do a lot more than just rear wheel and static moves. If you're leaning towards a streetier style with bunnyhops and manuals being part of your moveset then definitely look into the Hex. Much stronger than today's pure trials frames, and can do pure trials moves very well. Probably the best bike to start out on now if you want to explore what style of trials you want to get good at since its geometry allows it to ride well as both a big BMX and trials bike.
Take a look through the For Sale section of the forum, you'll find something good within your budget. Starting out on an older frame with lower BB can be better for the long term as it forces you to learn proper bike handling and control, and then you'd feel the benefits way more when you eventually move onto higher BB frames. If you find a bike you like, post it up and I'm sure other members of the forum will give you their pointers and advice on what's a good deal and what's good to learn on.