When I did Practical Pistol, almost everyone - including me - used Tokyo Marui Hi Capa 5.1's in 6mm bb, using 'green' gas (actually we tended to use higher pressure 'black' gas), modified to one extent or another. A few people tried CO2 but generally it was expensive in caplets, a bit of a hassle, and most of the time ended up smashing the guns to bits. TM hi capas have a vast aftermarket spares industry and are easy to work on. Priority modification is a metal slide because the plastic one will crack if you are using black gas. I liked to keep mine fairly stock-looking (although I did short-stroke it), but some people spent....a LOT of money. A lot. TBH you don't have to, a lot of it is in the mind; Reliability is the most important thing.
At one place I shot practical you could have used a pellet pistol, but if you had done so you would have been at a distinct advantage when it came to speed reloading and pace of fire. BB guns are plenty accurate at the range and for the size of targets you shoot in practical.

For IPAS I used an Umarex S+W 586 - perfect if you have a slightly larger hand. Also a slight advantage in that it is 10-shot. Most other people used a mix of CP88s and 586s, with the odd Umarex 1911 thrown in. Again the Umarex pistols have been around for years so there are plenty of spares available.
Only went to a couple of IPAS matches where BBs were allowed. If outside they suffered greatly in the wind and at longer targets, semi auto was not a massive advantage as far as I could see.

Doing something like IPAS or practical sorts the men from the boys when it comes to pistols. You'll put a hell of a lot of shots through in one day or evening, equivalent to months of normal plinking duty. Anything with reliability issues quickly falls by the wayside. The above will work, a lot of guns won't.