The starter on a A4 takes very little power - probably could start it with 8 D cells. (heck, that's funny enough I may try it and make a youtube of it)
A 27' boat with not a lot of electrical loads - two 'deep cycle' 12v batteries that fit in the boxes as long as they're bigger than G24s.
Some reasoning.... I have an "everything" (house) bank, and an "emergency" or reserve bank. Just worked out a bit easier than start/house and I kept my A/B/both/off switch. I really didn't have the room for the right switchgear to go with a dedicated start configuration although I agree that's the optimal setup. House is a pair of GC2s that fit in the original location. The reserve is a small G24 that I have actually never used - it's only there in case I draw the house down to nothing and it's kept charged with a tiny battery maintainer. So I start my A4 off my house. I charge the house from the alternator, charger, and 120w of solar. If I didn't have room for the reserve battery I would have just kept one of those little jump starters on board. I can go 3 days or so on the hook (I have refrigeration) with just the solar and still be able to start the engine fine and keep the beer cold. With the fridge off, I could last forever off the solar.
Without refrigeration, your power needs aren't gonna be too much. You can get 200AHish capacity with a pair of G27s. Or 225AHish with a pair of GC2s wired in series (if they'll fit). Your alternator will not charge quickly. Mine sure doesn't. But it'll keep up with the autopilot and plotter when motoring.
There is absolutely no reason for you (or almost anybody) to go the extra $$$ for AGMs, gels, LiFePO, carbon foam, etc... Plain old FLA is fine for your use. Buy em anywhere but west marine. (Costco/Sams are the bargains, particularly for GC2s). A G27 isn't generally as 'good' as a deep cycle battery as a GC2, but you'll likely never know the difference unless you put in a fridge or a 200 watt stereo or something.