Non-XR2 and XR2 shell have just one difference: the rear arches.

When you want to drop a XR2 in, get a engine with all parts. You need the top engine mount and the gearbox cradle out of an XR2. You also need the front CVH tie bars and it's useful to put in that rear anti roll bar and the XR2 panhard rod.

XR2 parts like the gearbox cradle, the tie bars, the arb and the panhard should always be used for stronger engines.

FRST is more work, it's got more electronics and you've got to make space for parts that were never fitted to an FBD. Things like the air filter box, the intercooler, the turbocharger with downpipe and so on.
ERST parts are a little bit bigger than FRST parts, but can also be fitted.
Zetec is also possible with some minor work on the top CVH engine mount, and with the whole 1.8 oil sump (when you want to use a 2.0).

Just check out www.xrtwo.com