Hi Mike,
My 18i8 (2nd gen) works great and is complete overkill for my (or your stated) needs, but hey room to grow, I always say.
1.) On the Roland Modules I’m familiar with, the Master Outs (assuming there are 2) can go to Line Inputs on the back of the Scarlet 18i8, 8i6 or 4i4 and that will record audio from your drumkit either on a stereo track or two mono tracks, or to a single mono track from the Left Master Out.
2.) You can run your guitar direct into one of the 1/4"/XLR Combo jacks on the front and set it to Inst in software (thanks to Geoffrey Bennett as mentioned by @GuntherT ) (The 4th gen solo and 2i2 got rid of the combo jacks and put the instrument ins on the back, so that can be a bit confusing)
3.) Run your mic (for vocals or mic’ing an amp) into one of the Combo jacks on the front (you’ll need to hit the 48V phantom power button if you’re using a condenser mic, not for a dynamic)
If you want to add a bass guitar later, you can always unplug your guitar. Contrary to the suggestion by @roax1, I really don’t think you’d want to plug your bass directly into a line-in on the back of the Focusrites. If you have a DI box or an amp with a line output, then fine, but not direct. I’d be happy to be proven wrong.
The Scarlett 4i4 and 8i6 will do all that, plus both have MIDI input in case you want to use your drumkit to play drum plugins down the line.
If you REALLY don’t want to swap cables between your guitar and your bass (or maybe bring someone in to play with you), then yes, a 18i8 will handle that.
FYI, through the Focusrite website (in the US at least), you can usually get previous generation or refurbished units at a discount. My refurb has been rock solid.
Good luck,
-Manuel
PS-I’m running Mint 21 as well