My car came with M030 both bars, which at the time was 26.8mm at the front and 20mm at the back. The 30mm bar was only fitted to the 968CS.
If you change both ARB's and want to do the minimum then you'll only need to change;
2 bushes in the middle of the front ARB
2 bushes at the ends of the front ARB
2 bushes in the middle of the rear ARB
Recommended Retail Prices for Powerflex bushes are here,
http://www.powerflex.co.uk/products/968+%281992-1995%29%2C+944+S2+%281985+-+1991%29-225/1.htmlThe more bushes you change the better because the ones on you car are probably 22 years old and way past their best, but anything else is optional. The 968 castor mounts are expensive and if I hadn't bought them about a year ago I'd probably just have bought the D shaped bushes for the 944 mounts. However Porsche do recommend they are fitted if you are running wheels any bigger than standard. If you change the wishbone bushes and/or the castor mounts you really should have the tracking done, so change all the front bushes at the front to save you doing things twice.
You should be able to take the front ARB off, which is only 6 bolts and do the wishbone bushes one side at a time, then fit the new ARB and bushes. I'm not so sure about the back because I'm not changing mine I haven't really looked at how that is done. The only thing you'll need a garage for is the tracking or you could take it to someone to do. I am also planning to take the cross member, rack and exhaust off to do the sump gasket, but you should be able to do just the front bushes without a garage.