I got a crown graphic, but that's because I wanted to eventually use wider-angle lenses. The crowns don't have a focal plane shutter, so you can pull the lens in closer.
One of the really neat things you can do with the Speed is use very old (100+ years old) portrait lenses without shutters, since you can use the focal-plane shutter.
Unless the camera's been CLA'd recently, I'd recommend that be first order of business -- especially if you find someone to clean up the rangefinder for you. Do you have a side-mounted rangefinder or a top-mounted one? The side-mounted ones are generally adjusted to one specific lens. You can re-adjust them, but you've generally got to pick which lens you want to use the rangefinder with. The top-mounted rangefinders use dedicated cams for each lens.
If you find you like the big 4x5 negs but the Speed isn't your cup of tea, you might consider a Littman 45 like blansky has (expensive, and I still can't wrap my conscience around modifying a classic piece of gear) or a 4x5 SLR. Graflex made several models, and there's a guy on the net (I'll dig up his site or someone else will if you're interested) whose really into them and uses them well, or there an outfit that advertises in View Camera magazine which makes contemporary 4x5 SLRs.
WRT kid shoots with 4x5, that's actually one of my goals, but I'm not quite there yet. I'm continuing to practice with our own 2 1/2 -year old, but it is a different type of shooting. With 35, I can explore a subject and think out loud with the shutter release. With 6x6, I tend to be a little more conservative and hold off if I see any nogos in the frame. With 4x5, a picture's got to WANT to be taken. I've got two grafmatic backs, each of which hold six sheets. Some folks here have close to ten. If you like the speed at all, I'd also recommend getting an eBay grafmatic or two. That'll give you about half the shooting speed and number of frames as a hasselblad w/120.
I'm probably sticking with my Crown Graphic until such time that I can afford a nice Linhof or two. It'll be a while.