In theory the main things would be:
- CFM flow rate / pressure drop at desired CFM (you can calculate this based on turbo size, boost pressure, and some assumptions about air temperature)
- heat exchange capacity (correlated to size and surface area of the IC)
- fitment in the car of the intercooler core and the plumbing for it
In practice, the #3 concern above (fitment) will be the most important.
Any intercooler will help as long as it is of reasonable size, so you want one that won't be too much of a fight to install and plumb pipes to.
Good options are abundant on ebay for aftermarket types, but the factory Volvo intercoolers as used on the gasoline (or later Euro diesel) 700 series turbo cars are also good intercoolers, nice and large, and of course bolt right in with factory plumbing. Personally I would probably try one of those first if one were available.
What year is your 740? There are 2 styles of factory intercooler, 1991-older and 1992-newer. You would want one from the same type as you have now.