Not to muddy the waters, but I see some erroneous or incomplete recommendations here based on my read of your questions and goals.
My understanding is that you'd like to run conventional engines with your existing transformers while using a handheld device for remote control. For that, you do NOT need a trainmaster command base. The command base is needed to broadcast the trainmaster command control (TMCC) signal for modern engines outfitted with command control electronics. Unless you own TMCC engines, you do not need a command base.
To run conventional engines with a handheld, you need: (1) Lionel powermaster, one per power block/loop, which can be bought aftermarket for $20-$30 if you shop; (2) CAB-1 handheld remote control device; and, (3) powermaster cable. The cable connects the transformer to the powermaster and normal track wiring connects the powermaster to the track. This enables you to control track voltage with the CAB-1 remote. CAB-1 can be purchased separately from the command base (you don't need) but they are easier to find together.
I have this setup on my permanent layout as well as my seasonal Christmas layouts. It's really simple and not too expensive.
As others have suggested, you could purchase a TPC300 or TPC400 and those will give you additional features and room to grow. I believe, but am not certain, they will allow also you control switches from the CAB-1 handheld. I've chosen to switch with old-fashioned postwar remote/toggle controllers for my switches and just use the CAB-1 to run the engines.
A modern ZW transformer comes with the powermaster capabilities built right in, so if you're contemplating a new transformer (I haven't but my brother-in-law did) then all you need is that and the CAB-1.
