When you say they don't keep long do you mean the working solutions or the concentrates? I'd figure on doing everything one shot. I spent some time with the Kodak data sheets a while back.
I ended up with the following feelings:
1) The 500ml Agfa kits don't make sense. Works out to be about $5 a 35mm roll. Considering that's about what getting it done by a good lab would cost it's not worth it. Even if the equipment was free.
2) The 5litre Kodak kits didn't make sense either. Not sure why. Maybe the life of the chemicals.
3) The 1 gallon concentrates worked out cheap enough that I could forget the cost of chemicals. The only issue would be the life of the concentrates.