If you want to give it an overhaul then you're up for gaskets, bolts, head skim etc to get the engine back to its original output.
If you want more ponies, then high comp pistons, new cams, larger valves & port grinding, new rings, gaskets plus a remap / new DME etc etc etc. Be prepared to replace every o-ring, oil seal, hose and spend a LOT of time cleaning and polishing. Brother in law recently rebuilt his Audi 2.8 V6 using 30 valve heads etc, new pistons cost him well over $1500 before rings.
If you just want a reasonably cheap bump in power go turbo. NA power can be had but good God it doesn't come cheaply. Probably the best bet is to start with another engine so yours is still on the road while you do the overhaul. Bear in mind that if you choose FI then you're probably going to need an uprated clutch and possibly a stronger diff as well. A lot of cash can be saved if you can weld / have a friend that will do it for you.
As I've learned in the last year, any increase in oomph requires a lot of time and quite a bit of money. There are a number of threads around documenting various M10 rebuilds, spend a bit of time researching just what can be done and make a decision on your goals. Ie 300hp NA M10 is just dreaming, but with a turbo it'll be doable.
Engine swaps are easier, but I personally wouldn't bother with anything less than a M52B28. Sohc just ain't worth the effort unless you have your heart set on one
Good luck dude!