Agree with Johnny. Brushless is practically maintenance free, it would be quite an effort to keep 4 brushed vehicles going well. And lipos can be charged right away after running, you can get by with just 1 per vehicle. However they do take a while to charge, I would recommend 5-6 for your 4. 2 per vehicle and you are really set. Old school nimh you would need around 12 packs. Just be sure to use lipo chargers!
Oh, suggest 'hardcase' lipos for safety and ROAR racing legality. Not all tracks require it, but I think its the way to go.
Another big change is the no crystal 2.4GHz DSM (aka Spektrum) radios that come with the Desert trucks. That will make life much easier with so many vehicles. And short antennas are better for the kids (big and little ones).
Good luck, welcome back.