I have rectifier charge systems on all of my motors save one.
I'm also a pretty cheap &*&(*((.
Walmart carries a group 24 Marine starting battery that is about the cheapest "marine" battery I have found on the market (at least on my side of the border). It's a heavy plate, lead acid battery well suited to rectifier systems.
They tend to last me 4 to 5 years under somewhat extreme conditions - 90 degrees in the summer, -20 in the winter with the battery sitting in an unheated garage (albeit connected to a "smart" trickle charger).
Batteries are rated a couple of different ways to further confuse things.
A battery that has 500'ish CCA (cold cranking amps @ 0 degrees) would have a rating of 600 MCA (marine cranking amps @ 32 degrees) or 750 cranking amps @ 80 degrees.
Since most of us only boat at temps above the freezing point of water, whichever battery you choose will always produce more amps than it's rating.
And for your particular model, the higher authority (Merc service manual) states it needs a minimum of 350 CCA with 100 minutes of reserve - which is about 450 MCA.
The battery I use from Walley World is the EverStart Lead Acid group 24-DC rated at 690 MCA (was made by Exide before they went bankrupt, don't know the actual mfg anymore - East Penn would be my guess) - more than enough