I have a '98 phase 1 red Spider as well! Mine has EGR (naturally blocked off by carbon buildup). I have two radiator fans and I can turn them on using Alfadiag - no radiator sensor - so as someone already said, a replacement radiator will probably need to be supplied with a threaded plug.
Meanwhile the dashboard will be a nice project when you get to it - do a search for 'centre console refurb' for inspiration... I found methylated spirits got the coating off, then it's your choice of matt black, dark grey, gloss black, or... I painted mine bright gloss red!
The radiator fans have a low speed resistor in a place very prone to damage on the fan cowl. Mine had the wires broken off. Then, the fans will not engage on low speed, so the engine temperature will continue to rise. Perhaps that combined with the weakened radiator explains the overheating/loss of coolant.
All you can do is tackle the obvious things - the radiator, the thermostat (which is probably not the cause of overheating but is usually faulty-stuck open) - and make sure the fans are working. Check the heater isn't leaking (mine was). Then see how you go - it might be fine!
On the other hand, mine had a blown head gasket and had this for some time. Changing the head gasket is a job that involves taking the cambelt off, so if that needs changing, it's the time to do it.
I know it may be daunting but since the body and paint are in good condition, it's worth saving, and the work you do will pay off in reliability and resale value. Mine has only average paint and body... and I had to fix a broken reverse gear, too...
The head gasket job was not too bad apart from a bracket under the inlet manifold which has two hellishly-difficult-to-get-to 15mm bolts. After undoing this bracket, I took the head off complete with manifolds still attached, which proved much faster than the last time I took the head off a TS. It's worth getting the tools for the cambelt change too (much harder without) - and other than that, it's easy. I didn't touch the valves or the engine bottom end, and it's running sweet.
So one thing at a time - and best of luck!