If your only requirement is to use the battery for powering the mover then a starter battery as said is suitable, however you need to recharge it promptly after every time it has been used.
It will be more readily damage if after hauling the van into storage you simply walk away,than would be the case with an identically rated leisure battery.
To minimise the % drain a mover takes and hence in time the damage it causes, then logically the bigger the battery is, the better it endures it. So if the mover drains 5Ah, this represents 5% of a 100 Ah battery, but a more damaging 10% of a 50Ah battery.
This largely guides the battery size selection till it gets simply too big physically.
I would not go below an 80Ah if I could not get it fully recharged in 24hrs, so it is not being put to bed below 90% fully charged.
For years I powered my mover whilst at home with a second 60Ah battery, to avoid large current drains on the caravans high performance GEL battery. This 60Ah battery lasted over 10 years but it was back on a smart charger in minutes after powering the mover, it never got left anything but fully charged..