Hi Chris,
I run a 924S as a daily, 20 000 miles a year all year round. It started as my "winter porsche" but I enjoy it too much so drive it all year....
The rule of thumb usually is: get the price of a very very good condition 924S with similar mileage, and subtract the cost it would take to determine the value of your car.
A 1987 160 hp 924S is quite a rare car, as it was fairly overpriced back in 1987 and it was an old car, full stop.
So, a good condition 1987 car would fetch at a dealers £2800 to £3995 (exceptional), subtract the above and anything cheaper and you have a bargain.
I am not sure what nougat brown looks like, but if it is espresso brown, or something akin to the cassis metallic that is found in the 911 range, than this is traditionally not a popular colour. Which might work out well at auction, price wise.
The waterpump/belts/tensioners (and runners I hope!) replacement is good news, that saves you a good £350 to £500 in parts and labour.
The car might need additionally a decent service (plugs, HT leads, distributor, all filters, oils (inc transaxle), brake fluid and servo fluid. (count another £375)
Geo would require the porsche tool to set the suspension at the rear, so do expect either to buy it at £15 or therabouts from your OPC and take it along, and anything from £50 to over a £100 at a specialist. The geo set up is money well spend.
Tyre wise, I run only the best conti's in 196/65/15/ V rated, which were something like £60 per corner- which is absolutely a bargain: lets call it £260 fitted.
Exhaust back box: £125
B post repair (that sounds like sills to me- you can "patch and repair" but really you ought to consider changing the sills) and mopping of the paintwork and a couple of layers of decent wax whilst-you-are-in-there: £600 to £750.
Refurbishing 4 wheels (is it really necessary or could that be done after winter?) is typically £45 to £65 per wheel, so lets call it £250.
Sundries like windscreen wipers, a battery for the key head (s) or a spare key you take on the chin.
So, lets say £2800 minus all the above (but not with wheels refurbishing) and you come to something like £1190. Anything below that is money in the kitty.
My car consumes 32 mpg, goes like stink, holds the road like a good one after suspension and tyres refresh, and costs me over 20 000 miles per annum 32p per mile considering the petrol prices. 32p per mile all in. Cheapest car I have ever ran.
Rather than refurbishing your wheels, consider buying a set of 6 x 15 teledials and run a set of winter tyres. (your 1987 should have 6 x 15 at the front and 7 x 15 at the back)
I have surprisingly good ground clearance and I always embarrass modern machinery in the snow. Over the german autobahns I am restricted to 190 km/h (118mph) due to my T speed index n my winter tyres, but hey- great fun overtaking a Q7 at 185km/h in The Fatherland.
Make sure the thermoswitch is working super well, and that the radiator is clean of gunk and chalk etc. This is what killed my engine the first time and I have learned my lesson. A proper flush, good quality fluids, and regularly looking after the levels.
IIRC the 160bhp is a high compression version needing 98ron so that means super unleaded, which is a pain in the wallet. I would consider calling Wayne Schofield for a remap. The 150 bhp version requires 95 RON, all unleaded.
If you have the turbo seats and your bum is not of the wide variety so you sit "in" it rather than "on top of it", than that is definitely a bonus.
My local independant has a white 924S in for ...£495. Can't believe prices like that sometimes.
Best of luck,
Bert