Here's a webpage (by a board member) that shows the various 200/700/900 wheels that will mate up with your 240 car. Note: The later 960 wheels (1995+) use a different offset that is not compatible without the use of wheel spacers.
200 Wheel Pictures
I've got a set of 15" Omegas from an '89 760 GLE on my '87 240 Wagon. They look sharp and the Omegas are a very strong wheel (like the 14" Dersus on which I have my snow tires mounted).

Here's a discussion on the strongest/weakest Volvo Aluminum wheels:
http://www.brickboard.com/RWD/index.htm?id=638075

This is the start to Paul Grimshaw's online information about Volvo Wheels. There's more material in his Technical Archives pages, and even more if you buy his books on Volvo Performance and the Gothenburg Bible.
Start of Wheel Section
Paul Grimshaws Technical Archive Menu Page

More info on tires, wheels, and sizes:
Wheel weights, specs, etc, for 200/700/900, and some FWD
Tire Size Calculator, for comparing different sized tires and determining speedometer error.
Wagon tire size info (Brickboard discussion thread)
Snow tires (Brickboard discussion thread)

Hopefully the info in the above posts should answer your questions, and probably spark a few more...
God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 228K