Welcome.
Getting a low spec Golf is a good starting point, but I think you've got your insurance groups mixed up.
The Mk5 GTI that your friend has, is not insurance group 8, it's group 17.
See here:
http://www.parkers.co.uk/cars/specs/Detail.aspx?deriv=28653As for the Mk4, here's some specs (copied from Parkers.co.uk cos I got loads of time right now):
1.4 - 75bhp - insurance group 4
1.6 - 100bhp - insurance group 5
1.6 16v - 105bhp - insurance group 6
1.8 non turbo - 125bhp - insurance group 10 (labelled as a GTI but most agree it's too slow to be a proper GTI)
2.0 - 115bhp - insurance group 10 (sometimes labelled as a GTI, but again too slow)
1.8Turbo - 150bhp - insurance group 14 (standard GTI)
1.8Turbo - 180bhp - insurance group 16 (GTI with higher spec engine)
2.3 V5 - 150bhp - insurance group 15
2.3 V5 - 170bhp - insurance group 15
2.8 V6 - 204bhp - insurance group 17
3.2V6 - 237bhp - insurance group 18
So as you can see as a learner/new driver anything above the 1.6 16v and you'll be screwed on insurance.
The 1.6 should be fine for a starter car anyway, you don't want too much power till you've got some real driving experience (ie after you pass).
A friend of mine passed his test and bought a little rear-wheel-drive sports car with a modest engine, but didn't have the experience to handle it and within 3 weeks had spun it and written it off.
That said, you should enjoy driving your own car, having the freedom to go where you want, when you want. So if you want a Mk4 go for it.
Take a look at this thread to see someone's experience of the Mk4 1.4 Golf
http://www.golfgtiforum.co.uk/index.php?topic=120673.0