Yes to the question of storing tank solutions between sessions. I never throw them out and start over, as they should always be relatively fresh.
The 45a tubes havent been made in a while but they are somewhat available, and last a long time. I have the set (used) that came with my first head maybe 3 years ago and they are still working fine, after hundreds of prints. I have yet to change one. Usually the heads can be found for 100-200, which is the value of the tubes alone, and in that case the whole rest of the head is a free spare. There's a 35mm mixing chamber available to replace the stock 4x5 sized one, so if you print 35mm only, you can use that and extend the life of the tubes significantly. I think it is the best color head for beseler or really any 4x5 enlarger system I have seen. Others disagree of course but the 45a was the peak of technology at the time, and about $2600+ when new. The other beseler color heads seem far less reliable too (anecdotal evidence).