For a single mic on a bandoneon I'd use a condenser from about 18" away.
Choice of mic depends on your budget and what other uses you've got for it.
They needn't be expensive to get decent results. The condensers in my collection are an AKG c1000s (About £100) a SE Electronics SE2200 large diaphragm mic (now SE2200A about £150), and a pair of T.Bone SC180 small diaphragm mics (£120 the pair).
The first one I'd try in this instance is the AKG
The C1000s is to me a great all round recording and live mic. I swear by it. Lorna Lavelle did her whol album on one of these through a wee Behringer valve preamp. If you read the mic reviews on harmony central however you'll find people who think it's apiece of shit.
One advantage of the C1000s is that it'll work off a PP3 battery if you don't have access to phantom power.
There are lots of other mics on the market around the £80-150 price point that will do the job admirably too.
Look at products by Rode and MXL, and the rest of Thomann's in-house T.bone range.
After that you're into the more expensive lads like the industry standard AKG C451 at around £270.
I'm not certain if the difference in price is worth it when we're dealing with home recording on a budget.
More later. Work calls.