
Can't offer any personal experience of swapping boxes (well, I've only ever swapped a box like-for-like, put it that way) but regarding what engine I can give you some info.
Unless I've got my thick head on, drifting isn't
per se about bhp. You can drift a Morris Minor with the right driver. Now, if you want to actually go round a track in under a certain time, with the tail out, of course that's when power comes in, and the world of 1000bhp Skylines etc appears, but given you're on a budget I'm assuming you're not looking at that level of tune

It's more about tail out weekend fun for pence than speed speed speeeed, yes?
All the V6 are good. They're the same basic engine, with 'bits' added or removed. By no means comprehensive...
2.5 / 3.0 the earlier version, (verrrry nearleeee) the same as the Calibra/Cav/early Vectra engine. Lots of Calibra boys out there with lots of mods they do. They are notable
by and large by their 'short plenum' (look it up) have visible ICV and EGR valves. Proper cable throttle. 6 x proper HT leads, one coil pack.
2.6/3.2s were
not made to make more powerrrrrr. They were a compromise of the engine to get around new emissions claptrap. The displacement was increased, but the compression was reduced. It is said they are worse on fuel than the earlier engines. Not perhaps a massive issue for yourself. Notable by their 'long plenum.' Electronic drive by wire throttle. Each cyl bank gets separate coil packs.
The later engines got smoother flowing exhaust manifolds. Most(all?) 2.6/3.2s did away with the EGR and ICV.
The 'big' version of each, so 3.0 and 3.2 got the high lift 'G cams' as fitted to the Vectra Supertouring and GSis 2.5. Adds a theoretical 20bhp to the 2.5 Omegas when fitted.
I'm biased, I own a 2.5. Personally I'd say get any of the V6s, but you don't
need the 3.0/3.2, don't feel you're short changing yourself by getting the 'lesser' V6s. But I would say that.
The theoretical perfect Omega engine is a 3.0, manual, with the egr removed, free-er flowing 2.6/3.2 manifolds, plus a few other bits here n there. Some say fitting 4 x G cams gives more grunt, others argue gives a wobbly power delivery (probably true)
There is lots above I've ever simplified, and some I've deliberately ignored and some I've no doubt inadvertently got wrong, but hopefully to a 'beginner' this will prove useful.
