You will likely find that you have a lot more good Roombas than you have good battery packs.
One thing important when revisiting the robots is to not try to charge completely dead batteries in the Roomba 4xxx, you can overheat and blow circuit board components.
Instead, first use a 12 volt power adapter or charger and connect positive (+) to the outside top slot and negative (-) to the inner top slot of the battery, and pre-charge the battery for a few hours to get them up to a working voltage the Roomba can deal with.
Actually, not that simple to rebuild batteries. The battery case has both security screws and the plastic is ultrasonically welded, so cracking them open takes a chisel in the right places with some experience, and then the 12 Sub-C NiMH batteries still are about $40 for a quality set (you get what you pay for). You then have to prep surfaces and solder with fast high heat to not overheat the cel, if you haven't made a tack welder. One interesting fact is the higher mAh rating batteries often have higher internal resistance, so you actually run less time with high loads like Roomba (and the rating is more often a lie).
Even though I have a pretty expensive battery reconditioner that can also measure NiMH pack capacity, it is still tedious to refurbish marginal old battery packs without just replacing all the cells - charging them up, identifying bad cells, measuring internal resistance and matching the best cels from several packs. I also have several more 4000/400 and 500 series than I have battery packs to go in them.
There's many revisions of both software and hardware in this series. I would generally recycle ones that don't have a pivoting front wheel, as they will also have old firmware and are missing other hardware revisions too. I have a scheduler remote
to get them to at least the scheduler firmware, if not the V3 OSMO firmware. Also discard chargers that are not "A" serial numbers, as they have a fault that can damage the Roomba in certain circumstances.
Then you just have to put together the best Roomba from the parts you have, using good wheel motors and tread, latest revision brushes and best gearboxes, etc, of course while disassembling and cleaning the motor and gearbox.
4000 series otherwise are pretty solid, they don't have known problems like the 500 series with IR emitters going out and hairballs melting the gearbox internals.
I'm in Portland, if you still want someone to go thru your roo-bots and let you know the condition of what you have and get some going 100%.