I have converted an early car fro track day use and I would advise the following:
Suspension: either new oem (as a minimum) or GAZ adjustable, with new (late type) bushes and perishable mountings. KW don't make anything for early cars. Later anti-roll bars will fit: in my opinion you should fit the thickest front part that you can find and then be prepared to experiment with differing thickness rear parts (Porsche struggled to make their minds up too and many different diameter parts were made). Old worn suspension simply can not hack it.
Brakes: new discs, new Pagid(OEM) pads (and NEVER EBC!), new hoses (ideally braided F&R) and new fluid as an absolute minimum. If you need more brakes then the easiest solution is to use later alloy Brembo callipers with adapters, which cost £80 - £100. You can fit later front hubs easily and cheaply (for bolt-on brake upgrades), but you'll lose your speedometer drive (race cars are frequently wide-tracked in this manner though...).
Trim: remove the cabin carpets, the hatch carpet and sound deadening mats, the door cards, the window regulators and motors, the rear seat back (the squab will provide a little sound insulation and it weighs next to nothing so some people leave them in) and the seats. Early seats don't weigh anything like as much as later electric parts, but there's still a good saving to be made if you fit one or two lightweight replacements.
Harnesses: if you choose (and they're not really required as a bucket seat will hold you in place), can be fitted using the early cars existing seatbelt mounting points. The rear seat lap belt mountings are ideal for the shoulder straps too and their angle will be relatively gentle (and always far better and safer than a silly harness bar, or a bar across the rear section of a cage too, for that matter).
Glass: the rear hatch weighs a lot, but you'll have to bite the bullet and pay for a thermoformed polycarbonate replacement. The side glasses are much more straightforward to replace: buy a piece of 1250mm x 1000mm 4mm polycarbonate sheet and make your own for about £31 (use a card template!).
No matter what colour your car is finished in, the cabin will look terrible after you've removed the carpets and sound deadening: I'd paint it white carefully, using the gloss paint that Series Land Rover and VW Bus owner enthuse about (google Jenolite and you'll find it).
.9mm aluminium sheet costs £25 for a 2500mm x 1250mm sheet. Use cardboard templates again and make a pair of door cards and a pair of footwell-linings. When the floors have been painted white and the linings fit perfectly, then I'd suggesting bedding them on expanding foam. It'll keep water out, it'll dull a lot of sound, it weighs little, but most importantly it will stop you from losing stuff underneath...
By now you'll have lost a lot of weight, but it'll all be off the back and the car will look like a Baja Bug (I ended up refitting the spare wheel to mine to try to redress the balance)! The only practical way to lose weight from the front of the car is by fitting GRP panels. A complete front end will cost about £600. I would certainly fit the bonnet: the rest is up to you. If you have original wings then there's a hefty saving to be made there too, because the sealer on them virtually doubles their weight.
Bigger wheels weight more: the ideal size for a 944 is 16", but the tyre selection available isn't fantastic. The lightest and best quality wheels that you'll ever come across are Fuchs, but you'll have to pay a strong price for them now. 7" and 8" with 225/50/16 and 245/45/16 are ideal, although the rear tyre size isn't nearly as well supported as it used to be.
Contrary to what some people will tell you, an experienced indy should be able to re-index your torsion bars in about three hours. However, if you drill a hole in line with the axle ahead of the rear wheelarch, you'll be able to access the torsion bar and re-index them yourself in about half an hour (or 15 minutes in a later car)...
Whilst they aren't compulsory; breather catch tanks, electrical cut outs and fire extinguishers all make good sense.
If you choose to only do one of the things that I have suggested, then it should be the suspension.