A retailer recommended battery will work for a while but you are using it at the limits of what it is designed to do. It will die after a few years (normally just as the warranty expires) and you'll have to buy another one which is what they want. Often they will sell you what they have and not what is best for your application - especially if they've got a surplus to shift.
My rule of thumb is to get the biggest capacity battery in terms of Ah (Ampere hours) and CCA (cold crank amps) that fits in the space available. Then when I need the battery to do its stuff under extreme conditions it can cope easily.
Ah = how long your lights will stay lit if you leave them on without the engine running. For instance on a battery rated at 100Ah it theoretically could deliver 100 Amps for an hour or 50 Amps for two hours or 1 Amp for 100 hours. Thats a very simplistic explanation but you get the drift. Most batteries cant actually do what they say on the tin unless under laboratory conditions.
CCA = how much current the battery can deliver while cranking the starter for a stated time at a stated temperature - normally 30 seconds at 0C. Some marine diesel batteries are CCA rated at -18C and are often cheaper to buy than your boggo standard auto battery. If they are good enough for the fishermen working off Peterhead then they'll do for me Defender down south and its what I have on my TD5. Check out the linky for details.
Numax CXV31MF Sealed Leisure Battery 12V 110Ah 1000MCA 500 Cycles XV31MF - Leisure Batteries - Numax Leisure Batteries
JMHO.
