The distinct differences in Mk1 handling are between the Williams and Valver.... Williams has better suspension, thicker anti roll bar and wider front track, very good handling and a lot of people use them on track with the standard stuff. Valver has a narrower front track and does not handle as well IMO.
Coilovers are not expensive for them compared to the Mk2s, as they do not have a rear spring setup, just a damper. 195/50/15 is usually the tyre of choice on 7inch wide wheels for the Mk1s, as it gives the best all round performance. The best upgrade you can do to a Mk1 (Valver) in terms of handling is fit the Williams (or 172 which is even wider) widetrack to be honest.