Road Tax exemption is available to cars built before 1 January 1973. It was originally going to be a rolling 25 year age exemption, but immediately it was introduced, Labour got into government, and realised that would cost them too much, so they applied the fixed date.
The SLC is similar in looks to the SL. It's also a R107 series. It was launched in 1972 with a 14in longer wheelbase than the 1971 SL. This gives it proper rear seats. Around 30,000 were built, and it was dropped from the range in 1980. When new, it was £2,000 more than a V12 Jaguar XJ-S. Consumption for the 450SLC when pushed = low teens.
They are considered less desirable than the convertibles, and are consequently less valuable. A minter should cost you around £7,000 (comparable SL around £11 - 12k). They are ideal GTs, but can also be huslted around pretty effectively.
As to buying; the same applies to the SLC as to the SL. Leaks are common around the bonded-in windscreen, door seals, non-Mercedes replacement glass and around the rear lamps. Early cars can suffer structural rust in the sills around the jacking points and in the box sections in the bottom of the engine bay - particularly in ABS-equipped cars like this one (ABS was an option until 1986). Post-1976, rustproofing was pretty good. Most bodywork panels and repair sections are available. It's a good idea to replace valve stem seals at 75k (£400-odd). Regular (6,000mile) oil changes are really important. Check out the radiator, it may need recoring (£150). Auto gearboxes are very reliable, and rear axles are also unburstable, but may be noisy. Check for a 'clonk' when changing from forward to reverse. Any transmission vibration on power take-up is a tell-tail of worn propshaft couplings.
Power steering boxes start to wear at 80-90k, and are expensive to replace, although much of the slop can be adjusted out. Sloppiness here can also be caused by the subframe mountings. With these cars, the original wheels (which it has) and correct tyres (Michelins or Conti's - I don't know which) will make the car feel so much better than non-originals.
Electrical problems come from lack of use rather than heavy usage. Heater controls and steering column switches can fail. Generally, after-market alarms are not easy to fit, and the loom needs to be chopped about, so check this area carefully. If the speedo flickers at low speed, it could indicate the odometer has been clocked, so check condition of seats, pedal rubbers, steering wheel and so on (not so easily done in a 'restored' car).
If the central-locking is vacuum operated, make certain it works well. Leaks are a bugger to find. Also, the reservoir, under the front offside wing can fall off if the brackets corrode. Obviously, check everything works, including the aircon. Lack of use dries seal out, then the system leaks and you've got wet floors.
Is the interior leather? (He says it is, so it probably is) MB-Tex is pretty well indistinguishable from leather, and wears better, but is less desirable (bargaining point?)
One key shoud fit everything. Pretty well all trim parts are available, but not cheaply.
The photos in this ad are not the best, but it looks like a straight car. Check condition and quality of chromework. If it's been restored, the silver bits will have been re-done, and this is not always done well. Price is towards the top-end, but if you're after one of these, and all the above items are sorted, then this could be worth a punt.
Hope this helps.
PJ
Oh well, I'd trust ME mag views more than a proggie I saw ages ago 
