Batteries aren't critical. Just buy the biggest that will fit, or, in my case, I buy the most economical VA/$. Here in the States Costco sells 115 VA (volt-amps) battery for about $80. If you can find deep discharge (usually labeled "Marine" for some odd reason, and they're not really deep discharge like the glass batteries in the telephone switches) batteries, that's the way to go (because starting a car doesn't really discharge the battery very deeply, but you'll often discharge a battery down quite a bit in an Airstream).
Never discharge a battery more than 80%--it eats the life out of it right away.