Obviously best wishes for the treatment and future first of all.
As others have said you have a lot of choice out there these days when it comes to Monsters.
Definitely go with the one you 'want' although the earlier bikes are showing their age now and can be a bit harder to live with (read more expensive if you have to put it into a dealers every time for fixing and servicing) unless you like getting friendly with spanners.
Really good ones are becoming harder to find and in particular the 900's are starting to fetch a few quid these days but there are still some bargains to be had- the 620, 750's and 800's are well worth thinking about for the earlier shape with the 620 and 800 being fuel injected rather than carbs.
From an ease of living with point of view, I suspect the 696, 796 and 1100 models would make sense being newer with various rider aids (ABS etc.) and even the lower capacity bikes push out not a lot less horse power than the bigger cubed early bikes did.
As already mentioned, insurance might be a factor to consider when looking at the larger engined bikes but a well set up and ridden smaller bike will still impress on the road which you may or may not be bothered about anyway.
To be honest any will put a smile on your face- the 1997 900 that I've owned for coming up to eighteen years makes me smile more now than it ever has plus there is a lot of knowledge and ownership experiences on here to draw from so don't be afraid to ask if something takes your fancy.
Oh... and there are some water cooled ones too apparently
(I actually had an S4 for a while so can get away with saying that- just, I think).