The starter battery is an N70Z from memory. The manual calls it an L3/760L. It's not a Nissan-only thing - if you don't like Nissan's price, you can get it from just about any battery supplier.
Dual battery choices are plentiful and money does play a part in it.
If you're charging from the alternator (using a simple isolator) then a Gel battery is a BAD choice. Gels don't like being charged at higher than 14.1V and the alternator should put out between 14.4 and 14.7V - the Gel will die. Generally, you can't start a car from a Gel deep cycle battery. Some Gel batteries DO let you start a car, but it has to deliver 550A or so to turn the 4-cyl diesel engine over (a little less than twice the 300-odd amps required to turn over a V8).
There are THREE batteries that you should consider from a deep cycle perspective. Ordinary starter batteries are no good because they suffer when discharged below 50% and if charged with the alternator, they're usually about 75% full - so you're really only playing with 25% of its capacity. That's not good.
Flooded cell deep cycle batteries are the cheapest option but unless it says on the label that you can discharge them to below 50% capacity, don't. Charged by the alternator they also only give you 25% of their capacity. Charged by a mains charger or a DC-DC charger, the battery will reach 100% charge giving you 50% of its capacity, but you will have to maintain it frequently.
This leaves the two types of AGM batteries as the best choice for deep cycle applications. The two main types are "normal" (flat plate) which generally are ONLY deep cycle, and spiral wound AGMs that can be used as crankers as well. I have an Optima D31A 75Ah 900CCA battery in my caravan ($549 in Autobahn, but $399 from
this site). It's bloody bullet-proof (pun intended ... they're used by the military). For the $ it would want to be.
FullRiver (Chinese) sell quite reasonable AGM (flat plate) batteries at reasonable prices although they are becoming more expensive as people find them more acceptable (standard supply and demand curve).
In my tub, I've got a 50Ah flooded cell battery charged by a mains charger (I invert the incoming 12V). We just went away for 2 full days and it ran the Engel fridge without a hitch for those 2 days. My only issue is that I have to keep topping up the electrolyte with distilled water. It's not ideal and when it dies I'm putting in a large-capacity flat-plate AGM.
