agree with both justyn and debs to a point... could be a very good buy, but we havent seen her in the flesh or driven her so its very hard to say.
"only light rust behind rear wheel arches"
all relative really but depends on whats meant by 'light rust'. easy enough to get plated, a local classic friendly garage who's handy with a welder would charge you maybe £100 - £150 to plate the rotten bits, but depending on the extent of the rot the repair mission could involve removal of fuel tank for welding, not a big job in itself but theres another hours worth of time on the bill. forget the filler and underseal, it'll rot through again inside 12 mths.
£1k or so for the lump replacement isnt bad if the labour included the supply of a brand new 0 miles engine, but at the end of the day the value of any dolly is mainly in the condition of the body. you can get hold of a complete set of running gear for a sprint 2nd/3rd or so hand for £few hundred in known working order.
no disrespect to justyn but a 1300 square headlighter isnt really ever gona be worth much more than £1500 for a complete minter with 12mths tax n ticket.
my view of 1300 price markers:
any car with 12mth ticket - £400 - 500
1300 in fair condition with 12mth ticket £600 - £700
1300 in good condition with 12 mth ticket £800 - £1000
1300 in VGC, no visible rot and 12 mths £1000 - £1500
1300 concors mint bling bling show car (showroom) £1500 - £2k tops
1300SE - forget it, you'll never find one, £3k for minter, I only know of one in existance!
these wont be far wrong and probably the root of some debate for the argumentative on here...
my opinion based on the description, pay more than £900 and you may as well get ya boxers off, bend over and give the bloke some vaseline into the deal.
if the car's that great get him to stick a fresh 12mths on it. it'll be easier for him to sell too. a lot can happen to a car in 6mths, and some carefully smoothed filler and underseal can easy appear to look like good solid metal, even if prodded, poked and tapped with a hammer, as I well know to my own poor judgement!
remember you're basically buying a pretty good shell with good interior that moves under its own power. 1300 is a pretty bullet proof motor and theres arguments to say it was the most reliable lump ever to roll out of canley, and a 1300 dolly is generally a great 1st car and/or entry level classic with cheap insurance... but while pretty good motors all said and done, 1300's (and to a slightly lesser degree 1500's) are worth next to jack all compared with 1850 and sprint as theres not much demand for them.
before you commit if thats your plan, keep your eye out for a 1500HL. theres more value in twin headlighters (even though the square lamps are actually better IMO if you fit halogens), and you'll also find there wont be much difference in the insurance.
young/new drivers are often easy prey for sellers, and its sometimes hard to tell how much integrity the seller has.
if possible get under the thing and have a good close look at the bushes, front and back (rubber bits on all the suspension arms, nearer the middle of the car rather than the far ends if you know what i mean). perished bushes are a proper pain to change. get your old man to take it out for a few miles as a test pilot, and give it what for too, not just a plod about. look where its been standing - any traces of oil or other fluid leaks on the floor?
be ruthless and relentless, lots of questions. numpties and blaggers may not have answers for all your questions and buckle under pressure, and make sure the answers sound consistant with the whole story.
im not trying to put you off, but with little experience it pays to have someone you know and trust with you. where are you and wheres the car? may be a club member nearby who could come along and have a look with you...