I'd be more worried about usage on the SSD than the battery. The battery is easily checked with coconutbattery. But I don't think there's a way to check the total writes done to the SSD from Mac OS (and even under boot camp because it doesn't support AHCI then)
You could get one that was used by someone to run loads of benchmarks, using up the write cycles on the SSD. Chances are slim of course but there's no way to check.
By the way I'm also looking for the 4GB-equipped base model (but the 11") but it is indeed extremely rare, especially at my local Apple Store (Ireland). I'm constantly checking the refurb store, they seem to be adding and removing stuff during the day. Especially at night there seems to be nothing available ever, not a single Mac.