Your batteries should be as similar as possible in age, size, type, condition, etc.
Reserve capacity isn't that relevant, I would look at amp hours instead. However, just about any group 24 deep cycle will be about 75 Ah.
If you have 2 that will give you 150 Ah. A fairly widely accepted rule of thumb for solar panels with no controller is 1% of less of Ah capacity. So, 1% of 150 Ah is 1.5 amps. That would be about a 27 watt solar panel. I would say 20 to 30 watts would be fine.
However, you can get a cheap charge controller for $20 or so. Then you could get a larger panel and not have to worry about cooking $150 worth of batteries.
