Think you'll have trouble finding an e86 with a 2.5, if memory serves me correctly they are all 3.0 si 261bhp and finding one for under 5K will mean it will be a shed on wheels!
The e85 vert has a 2.0, 2.2, 2.5 and 3.0. If you have set your mind on a 2.5, the early m54 pre-facelift has around 190 bhp, 5-speed box. As enzed4 stated, the facelift has two flavours 177/218 + a 6-speed box and a lighter engine due to the aluminium being alloyed with magnesium. Despite lower bhp and torque, it gives identical performance to the pre-facelift.
As far as I am aware (someone correct me if I'm wrong) the facelift 177/218bhp N52 engine is identical with the increase in power due to a 3 stage manifold and software tuning. You could, in theory, get a 177bhp and fit a 3 stage manifold (wiring already in place just blanked off), loads of manifolds on fleabay due to it being used on e46's and then have a remap. This would cost about £50-£100 for the manifold and £250 for a remap + you would need to be ok with a set of spanners. A good tuner may be able to squeeze a few more horses out of the engine, if you fitted a more free-flowing exhaust system I don't see why 225-230bhp is not achievable, so could be a cheaper way into si performance +.
If you can't find a decent 218bhp at the right price a 177bhp could also be remapped without changing the inlet manifold, this will bring it up to around 193bhp (according to all the remap websites) and again with a decent exhaust should see around 200bhp. Won't be as quite as quick as an Si but unless you are tracking it I doubt you would be able to tell much difference.
Avoid fitting a K+N unless you want to ruin the engine and reduce horsepower.
they ain't quick.
south yorkshire
