I'd be extremely surprise if you couldn't fit a ATX motherboard in a mid or full tower case. There is potential for problems if the motherboard you go with has a larger than ATX form factor (eg EATX or XL-ATX) but they tend to be limited to the higher end of most mobo manufacturers ranges. If you know what your case is you should be able to check compatibility (to the various board size standards) on the manufacturers website. Alternatively, check how many PCI slot cutouts you have on the back of the case - if you've got 7 (or more) a standard ATX mobo is virtually granted to fit.

there are 8 slots in the back so there should be plenty of space.

anyone pitch in with the "sweet spots in the current CPU lineup"?

You might find this useful:
Toms Hardware CPU Roundup (June)
If you're not overly bother about overclocking and have the cash the i5-6500 looks pretty decent (£185 or there abouts from Scan). I'm not convinced the i5-6600 is that much better as you only get a 0.1GHz frequency boost (0.3GHz Turbo) for an extra £15 when you can get the i5-6600K which ups things by 0.3/0.3 GHz (core/turbo) over the 6500 for an extra £30 (and will also let you overclock).