>1 can a b23/b230 from a 240 fit? (plus the m45 gearbox)

This isn't a particularly easy swap. Putting in a B20 is a direct bolt in swap and it fits right up to the existing tranny - and its easy to put in an M41 OD tranny as well for 5 speeds. B20's can be built up to a streetable 200 hp or so with some healthy spending.

Issues with the OHC installation:
- won't fit in slanted - so it must be installed upright. This means a B20 oil pan/oil pump swap - as well as a conversion to the bellhousing to allow the tranny to remain upright.
- the remote short shifter on the M45 (or a remote shift M40/41 for that matter) will require some surgery on the tranny tunnel

There are probably more issues that I'm not aware of - but this is certainly something that has been done qute a few times before.

>2 what can i do to upgrade the brakes (eg twin cicuit, front 240 brakes)

I have no idea on how hard it would be to fit 240 brakes on to the 120 suspension, but I do know that 1800E/ES brakes are basically 140 pieces that fit on 122 hardware. With enough 1800E/ES parts you could swap the front rotors, calipers, master cylinder/booster, and rear axle to get 4 wheel power discs on a dual diagonal split. The rear axle won't quite be a direct bolt in - but close. You'll need to graft one set of arms together with a 122 front and 1800E rear sections.

>3 i have a set of the 5 spoke gt/glt mags - the stud pattern is not the same can i make these fit without spending a fortune?

If you've done the above 1800E swaps then you'll have the later bolt pattern. Although the earlier pattern is very common - matching 60's Fords and Dodges and lots of other cars - so lots of nice looking wheels are available for it.