If it has both a Sprint gearbox and back axle this increases viability.
An 1850 with Sprint running gear will invariably be worth less than a Sprint in the same bodywork condition.
To be honest I think that an 1850 gearbox/axle would have a short life with a Sprint engine. They are pretty marginal with a TR7 lump, from the experience of other members.
My advice to the OP is to be very careful. If it is a project fair enough, that is to be encourage. My car is more about me fiddling with it, I will never make money from it.
However if you are looking for a bargain, I would save more and go for a better condition car that is running. The golden rule of classics, in the long run it is cheaper to get the best you can afford.
At least check the engine turns over, and get the thing out of the garage, to check the body properly. If you see any rot, walk away. They might sound harsh, but you will be far better off spending more on a car with a sound body.
As for values, look at what similar cars have sold for on ebay, you can search for completed listings. It should give you some idea.