I can't offer a wealth of experience as I've just got a comp 40x to replace a Big Nikko Mk1, so I'm about 3 weeks ahead of you.
I got a 4" wheel but if I was doing it again I'd go smaller, I see lots of 3" wheels and now I know why. The gap between 50 and 55 is bigger than the Nikko and on a 4" you could easily mark every yard if you wanted to, a 3" might be a bit of a squeeze but is probably achievable. You don't need to worry about small wheels.
9 years of 'see-sawing' into focus with the Nikko is a bad habit I need to break PDQ, you do need to come in from the same direction each time or face the consequences on the long ones! Focus is no worse than the Nikko, just different.
Sunshade by default. I don't know if it needs one but I'd rather not find out it does half way around a course for the sake of a few dollars.
Mine comes down to a little under 11 yards, the adaptor brings it down to a mega sharp 7. Advice obtained here (based mainly on Leupold Germany advice) suggested not to reparallax it.
Butler creek #39 for the scope and #30 for the close focus. I got mine new so just got the post 2004 adapter and put a 38mm hole in the butler creek and used sandpaper on a dowel to open it out enough to be able to cut a very tight thread with the adaptor. I've seen some where the lens has been removed from the 40mm shroud and is held in place by a very tidy looking housing, it looks a bit like a porthole. No idea where they got them from though. I'm not sure if there is any real difference between pre and post 2004 versions other than potentially the thread, but the post 2004 was in stock so just paid a few dollars more for the newer version.
Need to sort out the ret, probably will end up with the duplex post, not dissimilar to the Nikko NATO ret which I really like. I've never liked mildots, but appreciate it would give me additional aim points for close ones, so it's a personal thing for me as the general consensus is in favour of mildots. I suspect mildots would make it much easier to sell on if you didn't get on with it.
Hope that helps
ISP Airstream MK III No 15 which is a bit poorly at the moment and having a rest
AA Pro Sport + Ginb FT8 stock + Bushnell 4200 6-24x
EV2 MK1 + Leupold Comp 40x
Weihrauch HW100 KT laminate
SBFTC Competition Secretary