Using this page:
http://www.tyresizecalculator.com/tyre-wheel-calculators/wheel-offset-backspace-calculator
and assuming this is the standard size wheel for a Volvo 240:
14" wheel, 5.5" wide, +20 offset
standard volvo wheel backspace is 102.55mm
Back to the tiresizecalculator website, changing the declared wheel width to 7", and setting the backspace to 102.55mm gives a result of (roughly) +1 offset.
Keeping the backspace the same but going wider wheels means that the extra width is extending out further - it won't foul your suspension or inner wheel well, but depending on the tyre size you'll most likely need to roll or massage your fenders.
I moved my stock 14" wheels out 25 mm with spacers and it rubbed on the back.
If you get to choose what offet/backspace your new wheels will have, you should put a little bit of time into understanding offset and get it right. If they end up being too far in (offset to high or backspacing too high) you will need spacers to clear the suspension etc. If they are 'too far' out then you'll need to need to work on the guards to stop rubbing, but you'll gain track and they will look more aggressive.
I'm sure others here have seen good offset or backspacing numbers for 7" wide wheels on a 240 that can contribute.
