Oracle sells a tune-up kit for $150, includes new springs, belt, etc.. take a look on their website for complete info.
In terms of upgrading I think it'll cost way too much to bring your MK I up to snuff. Probably be better to buy a used MKIV or Mark V. As an example I found a used Mark IV with turbo supply, SME 5 arm, and Dynavector pick up for $2500 for my boss.