The easiest conversion for you is to get a early my94 to my96 WRX front cut. EJ20G turbo
Get it with the rear end assembly and everything will bolt right up. Minor electrical work to get it all wired up.
You can also get a RS Legacy Front cut. No electrical work required. Just transfer the whole Dash and wiring loom over. But Id recommend a WRX cut over the Legacy cut because the Legacy's have oil gallery issues in the head that show as tappet noise in the hydraulic lifters as the oil ages and galleries get clogged or blocked. It can be managed by regular oil and filter changes with quality oil and the use of additives. But you'll never really be able to get rid of it.
If your still keen on a 2.5L conversion its still possible to use a 2.5L block and bolt on early WRX heads and manifold (Legacy heads will NOT work).
If you use a late model 2.5L turbo front cut you are going to need extensive splicing of the dash and engine wiring looms or an after market
ECU to manage it all.