Please Recommend Better Product (Help a Newbie make a decision)

[Justice]

An 870, 88 and 500 disassemble in the exact same way.
An 870 barrel can go over $300(Cabela's Canada wants $339.99 for an 870 barrel. $289.99 for an M500/M88 barrel. However, there is tons of aftermarket stuff for 870's that is not available for Mossbergs. Dunno why.
"...My dad has..." Pop's LOP too long or too short? Assuming a wooden stock, both are easily fixed with a mitre box/saw and a recoil pad.
1) New stuff comes from the factory coated with a rust preventative. You need to clean it before shooting it, but not necessarily for immediate storage.
2) Depends on where you have the cabinet. Basements tend to be damp(not all of 'em of course. My Ma's basement was cool and dry. Temperature varied about 10 degrees all year.), but bedroom closets are not.
Most gun cabinets are not air tight, but a low wattage(25 watts) light bulb, left on all the time, will raise the temperature in the cabinet enough to stop condensation happening. The whole thing goes away if you put the thing in a dry place. Bedroom closets tend to be dry and gun cabinets fit in 'em nicely.
You'll probably have to drill a hole in the back of the cabinet for the cable. Drilling steel does not require any special tools. Eye protection is mandatory though. The size of the hole will depend on how and what you're powering. Smaller hole if you put the connector on yourself.