For live performance the absolute minimum I would consider is a 200w combo and even then only for shows at which you're sure you'll be running through the PA system... you'll probably have to run anything less powerful than that flat out which will sound terrible, especially with the generally poor quality of speakers in combos at the lower end of the price range, plus you lose all dynamics and headroom. 200w sounds like a lot on the face of it but most amplifiers are rated on the output of the amp when it is running a *4 ohm load*, so the output of the amp is 200W only when it is connected to a speaker(s) that is rated at 4 ohms. But the speaker that is built in to the combo is usually 8 ohms, which doubles the load and results in half the output (the maths is not quite as simple as that but that is a good rule of thumb). So the 200w combo probably only puts out 100w or so in terms of what's actually coming through the speaker. This is inadequate for live shows in my view (unless you're always going through a PA system, or you play coffee-house type background music in which case power is not an issue). Of course lower rated combos are hardly putting anything out once you take this into account. You *can* get 200W out of a 200w combo if you connect an 8 ohm speaker cab to the extension cab speaker socket. 2 x 8 ohm loads gives a 4 ohm load (no, I'm not sure why it works like this either, but it does) and the full 200w of the amp will now come through the two speakers. But if you have to lug another speaker around to be loud enough then you might as well not get a combo. I don't know what your budget is but something like the Ashdown 300w 1 x 10" combo might work for you, that puts out about 200w with its own little speaker which is enough for smaller shows and 300w if you bring along another 8 ohm cab. Roland also make a couple of smaller bass combos which appear to have a speaker matched to the amp built into them so you do actually get the full power of the amp out of them though equally I didn't much like the sound of them. And you can't add an extension cab to these ones either. Euphonic Audio do a great 300w combo that costs a fortune, and Ampeg do a nice little 300w combo too that is well reviewed and very portable. SWR's Black Beauty is good too (500w at 2 ohms!) and there are lots of others too. As ever, if you want something decent, be prepared to spend money. It would help to know what your budget is and what kind of gigs you want it for. If you're serious about music you'll end up buying something decent sooner or later so you might just as well buy it now and get it over with.
Anyway, the main point is that the combos you have in mind are underpowered for anything but low-volume rehearsal or coffee house-type noodlings and if you are likely to play bars, clubs, halls etc. with a proper rock band in full flight then you really need 300w and up. You might be better off with a separate 300w + head and a smaller 4 ohm cab in the long run.
Good luck, let us know what you decide.
