I would advise going the turbo route,
the reasons for this are, much better fuel economy, easier sourcing of parts, future expandability if you do decide you want more power.
Also another good option is a turbo'd 998, you can get a smooth reliable 100hp relativly easily, and due to the shorter stroke of the 998 you end up with a nice buzzy engine.
Basically its not massivly hard to do If you know one end of a spanner from the other, the main things you need to worry about uprating are....
Pistons, some of the std pistons arnt up to much so rather than plunking a turbo on a re ringed bottom end, get it rebored +20 thou with a decent piston, minispares mega's, omega, will all be up to the job, and many people run 180-200 hp on these. get pistons with somthing like a 15cc dish to get the compretion ratio down a bit ( more of that later)
Diff, the std diff is a bit pants, if your going much over about 80-90 hp you need to think about changing it for a X pin diff, get the minispares one, its proven to be nearly indistuctible,
Clutch, chances are you'll have to uprate the clutch, if you have the preverto style you can change the pressure plate to an orange (ok for about 120 hp) grey (ok for about 150-180) or double grey (200hp+), dont just go out ab buy the strongest one though! as strong clutches put a lot of stress on the crank thrust bearings.
If your running a verto, you can put in a metro turbo cover( good for about 120-130 hp), pull apart two std verto's and rebuild them into one unit with two springs( good for 200+ hp) or swap to preverto and follow the above, my prefered route is to use the verto as they are smoother and put about 1/3 of the stress on the crank thrusts ( read eliminate the problem).
Gearbox, you'll here people say you need to go straight cut everything, but this isnt really so, a good standard box that hasnt had a hard life ( ie one from under a 998 that has had regular oil changes) will be ok up to around 140-150 hp, the drop gears are pretty strong, and theres a chap that has been running a set of MPI ones at over 200 hp for years.
Compressin ratio, a lot of people will say you need to drop the compression ratio right down low with a turbo or it'll explode.... nope, aim for 8.5-9:1 and you'll have the scope to run about 15psi boost and make about 140 hp. chose your pistons, and then get a head to suit, Turbophil on here does good headwork at a good price.
that covers any engine mods really,
Turbo & ancileries, you have two options here, get the bulkhead chopped out to make room for the metro turbo setup, or get a mirage racing exhaust manifold and down pipe, the std metro is a T3, the original Mirage manifolds were made to take a T2, but I think they have now swaped to the more modern GT17. Picking the actual turbo is a bit of a nightmare as you cant just go by the model, (T2,T3, GT15,GT17, GT20 etc) as they all come with different compressor and turbine sizes dependant on usage and can overlap massivly, as a general rule though, most T2's are suitable, the gt17 form the saab 95 is about perfect, some T3's and gt20's are too big ( unless your looking to make lots of power).
you will need to run an intercooler, just get something that looks like it will just fit, wedge it in and pipe it up.... RS turbo, R5, 2wd cosworth have all been used to good effect, but the list is endless, the only rule is usually bigger is better.
fueling, use all the metro turbo stuff, carb reg, ect... but for ease and neatness rather than the metro fuel pump, you can use an MPI tank with the internal pump.
Diff ratio (as you mentioned it) dont bother with a 4.1 or somthing low, you'll find that 1st and 2nd are non existant and you'll loose more time swaping between gears than actually acellerating.... its generally accepted that a 3.1 is a good compromise for a turbo especially with a std box, some even like to go for the 2.95.
this is getting a bit war and peace not so I'll end it there for now.
