i fitted a new engine in my mr2 roadster, same engine as previous just a lower mileage version as the older engine lost compression on cylinder 1. and when i sold the car as soon as you said new engine people went as cold as a fish. was an absolute ballache to shift.
engine swap especially isn't that much work compared to doing something like an LS1 or FI install if its stock for stock install.
but when the difference between a 2005 276 and a 2007 313 is less than 1500
but you not only get the engine and gear box but the bi xenon headlights, LED rear lights, upgraded centre console and possibly lower miles all still on original parts.
it makes it a better buy to upgrade the whole car.
my guess is you'll be looking at the best part of 1500 for engine and gear box. you'd probalby get between 500-800 for your old kit that comes out. but if you factor in buying the other parts that make a 313 what it is. you'd need to spend another 1500 at least to get lights and interior.
abbey may well know of a better supply of engines at more favourable rates. but i still say its better to buy the upgraded car than swapping the engine