one bms you might want to look at is the chargery bms16, you can find on ebay for 100 dollars, its fully programmable. You can program the high/low cell voltage cutoffs. One thing it can do is trigger external relays, very important if you plan on using solar on 12 volt systems. You can put a relay in between the solar panel and solar controller, when it reaches your set voltage it disconnects the solar panel and stops charging.
If the voltage in the cells are common either li-ion 3.7 volts or lifepo4 3.2 volts, you can get away with using a cheap 6 dollar BMS that can handle 25 amps, I use them on my battery packs for years. If you want a BMS that can handle 100 or more amps they are in the 40 dollar range. The only problem with using these on solar systems is if the BMS triggers to stop charging, the solar controller will try to force a charge on the battery causing voltage surges. These surges are bad for any fans/lights connected to the battery.
BMS is not that complicated, you can buy the 4s balance connectors for 10 for 5 dollars, solder them to the battery and your set to go. You never want to overcharge any of the cells which will cause a thermal runaway. And if you have a battery that won't stay in balance, get some active balancers. 4s active balancers are also in the 100 dollar range. I have some on my 4s 220ah lifepo4 battery pack, they work excellent.
I been using an overvoltage relay (6 dollars) on my system for the past year. Its like a deadman switch. If the battery voltage ever gets too high it triggers a relay that disconnect the solar panel. You can never have too many safety features on your lithium battery.
A good website for building lithium battery packs is secondlifestorage.
