It depends on what kind of power you want (i know everyone says as much as possible), but also depends on how handy you are with a set of spanners, and the time you have. For about 150-160bhp, you wont need to lower the compression (although it would be better if you did) so you wouldnt need new head gasket, head bolts, and all gaskets around the head, aswell as getting it all timed up again. If you cant do all that yourself, that will save you alot of time and money!
So basically, youll a plate to attatch a turbo to your manifold (SonnyP can get you 1, not sure how much) a small turbo, like a T2 or T25, id use an intercooler, not 100% necisarry on low boost, but as youll probarbly go for standard compression, id reccomend 1. Youll need a bunch of silicone hoses, and some aluminium pipes (you can buy an intercooler kit off fleebay). Youll need oil feed and oil return pipes, powerboost valve, and a custom downpipe. If you go for the saab 9000 turbo like me, you can just modify the saab 9000 downpipe to fit, that will save you a fortune, as a custom downpipe will set you back at leaste £200, and youll find alot of exhaust places will struggle with it, as theres not alot of space at all. If you can do all the work yourself, you can do it for around £500!
But if youve not got alot of experience with engines, it will cost you alot more, theres a fair amount of work, and it could end up costing you alot. You need to remember that the more power youre getting out of your car, the less reliable it can become. If you want to go the whole hog, you can either buy low compression pistons (about £800 plus) or use a decompression plate (£40 plus p+p), obviusly low comp pistons would be ideal, but the de comp plate works fine for me. Then you can go for aftermarket management, youll be looking at about £300 for piggyback, then wiring, then mapping and rolling rd time, or £600 upwards for standalone management, then wiring, and mapping, or you can just use standard ECU, and manually adjust the fuelling, and play with the cam timing to get the power curve where you want it. Im going for megajolt (about $200 plus shipment from US), looks fairly easy to wire in, and should allow me to put the rev limit to whatever i want, remove the torque limiter, and run more boost by retarding the ignition. If your any good with wiring ect, then megasquirt or megajolt are the cheaper way to go, you can achieve as good results as the more expensive management, but a fraction of the cost, although, if you use megasquirt, you will loose the VCT. If you dont already know, mega squirt is a kind of DIY ecu system, and megajolt is made by the same people, but only controls your ignition timing.