Aye Steve,
Depending on the price, buy it! The wide case Scrambler Models are essentially the same as the wide case MKIII models, apart from the re*******at loop of the frame, the handlebars, seat and tank. I ran my 450MKIII on the Island as sole means of transport. Essentially the same as you have been offered, but with rear-sets and clip-ons.
You do not say what CC the beastie is, but a sound 250MKIII can produce just over the ton, the 350 similar and the 450 the same.
Spares for the motors are readily available but NOT CHEAP and DO NOT TAKE THE MOTOR APART IN A RANDOM MANNER!!!!! ALL SHAFTS/GEARS ARE SHIMMED AND THESE NEED TO BE KEPT IN THEIR PLACE AND ORDER!!!!!
Keep the oil clean, 1,000 miles between changes and use either a SAE40W mineral oil or a synthetic SAE10/40W oil. I prefer the SAE40W mineral, but many racers use the other or Castor SAE40W oil. The latter should only be contemplated if you are going to change the oil after every race meeting and doesn't mix with mineral oil.
DO NOT TREAT IT AS A 'THUMPER'! These motors will knock the big end out very quickly if treated this way. My 450 will not pull away cleanly on less than 2,500rpm and revs out to 6,500 - 7,000rpm. The 350 revs out to 8,000rpm and the 250 to 9,000rpm. When racing and using a desmo head, 250's have been taken to 10,500 - 11,000rpm. A Desmo head is only worthwhile on the 250cc or 350cc motors, as they can rev sufficiently high enough to take advantage of their valve control capability.
The electrics will need careful inspection and if they go phut will need replacement. The wiring is thin gauge and readily corrodes, as do the connectors. The electronic ignition failed on mine and upgraded to 12v electrics, points, coil and battery succesfully. In the current rebuild after 15 years off the road and a total of 75,000 miles on the clock, I have upgraded to a 12v system using an 180W alternator and may use points triggered electronic ignition and LED lights. You already have heard of the man in Onchan to whom I would direct you if you should require the electrics seeing too.
I am happy to provide more info if required.
My regards, Bill