Starting out with RBA's is a good idea. 30 watts to 50 watts is what I would recommend. You wont use all 50 watts at first but there will be some headroom there when you need it later.
I'm thinking something like the Sigelei 50 watt or IPV 2 or IPV 2S (The 2S is a slightly updated model or the IPV2, it's got different buttons and newer firmware I believe with some other updates/fixes over the IPV2).
The Sigelei 50 watt and IPV2 both use one single 18650 battery, which keeps the size smaller and lighter and it should fit nicely in your hand.
I don't recommend the Sigelei 100 in your case because I just don't think it's needed, and it's bigger and bulkier. The IPV2 has onboard charging built in via USB, I'm not sure if the Sigelei 50 watt does or not, I know the 100 watt one does not have built in charging.
I don't recommend the Cloupor T5 in your situation because I think you'll be better suited for your mod and RBA / RBA evolution with a mod that will have a broad range of resistance and power.
There's probably some other mods out that would fit the bill just right also, those are just the ones off the top of my head that I thought might be well suited and also are somewhat affordable if one's budget allows it.