I'm also about to pull a trigger on the ultimate so I'll do a bit of 'splainin.
1) 39mm version is not called jumbo, it's called regular RO. 36-38 ranges are all models off 14970, which is the predecessor of 15300.
2) 39mm RO without seconds hand is the current and discontinued jumbo (15202), of which no correct replica exists
3) I never saw a rep of 15300 in person, but from the pics it looks damn nice. Gens DO have a bit of glossy-ness and very discreet sunburst dial, but not as much as rep 15300 pics so you might want to add some matt paint to the dial. Indices/hands are also VERY nicely polished so you probably want to do that yourself too. Gen lume is [censored] poor too so why bother? This ain't a diving watch. No options for strap, not even for gen(ss). If you don't want to spend $400 in aftermarket parts then request your seller to sell bracelet +strap with the neccesary plots Sorry to say that I have never seen quality RO (not ROO) straps from any makers. Most don't make them, and those who make it sell crap for a high price. Go AP OEM if you must have that strap, b/c it's in a totally another world from custom strap, self-made, etc.
3) Where? Most trusted dealers sells them I think. Trusty and puretime has'em for example.
4) I would still wait to see 15400 released b4 making a decision, that's why I haven't bought one yet.