You'll get a lot of advice on questions like these based on people's actual gear - not many people have used everything on your list.
As a Speedo guy, I support that path. I have packs from the 1970's that are still going strong. But the 2400 is a lot of power - of course you can use one channel and get (I believe) 400ws to one head. I'm thinking that's not a dial-down pack, which is huge but not 100% necessary. You can always double up diffusion, or make some ghetto ND for open-faced lights (Square scrap of aluminum window screen, glue an "X" of coathanger wire to it and you can sort of fold it right over the reflector and use as many layers as necessary).
Service - You can send gear to Speedo in Chicago. Some big cities have a strobe repair service (we have on in Dallas) that are authorized for Speedo and have genuine parts. Only time I've ever needed a Speedo repair was when an optical slave ended up holding the test button down and quietly fried a pack. On location, when the fan-cooled heads get knocked over, the handle pushes the fan into the guts. We've all gotten good at taking the head apart and beating the can back into shape. That's why you see so many speedo heads that looks a little, umm, "deformed", but if the fan runs smooth, you're good.
Just one head is kinda rough, but eBay is usually full of them. Big sheets of foamcore and more stands can help with fill. Speedo makes a non-fan-cooled head that uses a 150w modeling lamp, is much smaller than the 103, and cheaper - they don't show up used as much but they're great heads too.
My favorite Speedo accessory - the one thing that really kept me in speedo gear - is the 11" reflector with a grid and gel frame in it. Killer for hair/cheekbones, killer for still life, you can stick a sheet of mylar behind the grid and use it on faces for a beauty-dish look. Just something about the size and light shaping of that I love.
Speedo's refresh speed isn't that hot for fashion, but can go fairly fast at lower power levels.
As far as synch voltage with modern cameras, there are adapters that are about $50 or so - but, the first gen of PocketWizard radio triggers show up used all the time, and they're awesome. The cheap Amazon Yongyou triggers are pretty good for $25, though my RBs and older meters won't fire them. Modern stuff works fine, all my older SLRs and 4x5 work fine. A radio slave is just about must-have in my opinion, once you use one you'll be like "screw those damn cables!!"
Fotodiox Pro softboxes (on Amazon) have come a long way - the quality of the ones I have bought rivals the big guys. A SIX FOOT TALL x 1 foot strip light, with a grid, for about $160? And the thing is butch. So check that out. Cheap chinese softboxes from eBay (many come with a speed ring for your brand) can be just fine if you treat them OK.
Keep in mind, a few yards of ripstop nylon from the fabric store can be clipped to a c-stand arm and you have an instant soft-light source, just not as controllable as a soft box. I've hung yards and yards of the stuff to make whole-room or entire-wall softboxes for stuff like the pic attached. Get a few yards of black felt while you're added for making strips or controlling things.