Generally you wont find turbo's on engines smaller than 1.6 or 1.8. I know it's a pain in the arse, and probably not what you want to hear, but turbo conversions cost alot of money and you'd be better of saving your cash and going for something with a little bit more poke as standard. The most BHP out of a standard 1.4 mk3 Astra would be 88 from the 16v engines (you didn't state which model you have).. Whereas the 1.8 and 2.0 engines could be anything between 115 and 130 BHP as standard, and therefore a much better tuning 'base', capable of up to 200ish BHP (I would imagine)..
If you are really keen on Astra's, either go for an engine conversion; or for alot less hastle just sell and upgrate to something like a GSi.
Spending around £4k on a Turbo conversion on your 1.4, you also need to upgrade suspention/clutch/gearbox/camshafts/driveshaft.. The list is almost endless..
Witha 1.8 or 2l engine, you could upgrade parts such as air filter, manifold, cams, exhaust.. And still find yourself with a fast car, for about half the price of a turbo on your 1.4.
