Yes, the solution is just saturated KCl, about 3.5 M. You can make it yourself.
You need to refill it whenever the solution fouls up, which does not happen very often, perhaps once a year with frequent use.
Yes, it is a good idea to buy a spare electrode. As for the junctions, it's up to you. I find with dairy, it is not the electrode itself that becomes clogged, but the junctions. The electrode you can clean from protein and biofilm by soaking it in pepsin or rennin, and by soaking in a HCl solution. But when the reference junctions are fouled up, it is not easy to fix them. The two types of electrodes I use have either special teflon or extended life ceramic junctions, and they hold up very well. Not sure what the Extech junctions are made of.
And don't forget to buy reference solution, or make it yourself.