Hi Kelly, welcome to TFK!
I believe Byron is the resident expert on planted tanks. I would just add that if the tank is medium to heavy planted, N2 cycling is nearly a moot point. The plants will use most of any ammonia generated so there is very little nitrites and nitrates created. So after you setup the tank and have planted, you can add a few fish right away. The key still is to only add a few fish at a time, allowing the system to easily stabilize. I realize only a couple of fish in a 55g tank seem lost, but I'd add only 2 or 3 or a small school of small fish per week until 'stocked'. This will better ensure success without stressing the fish.
I also would suggest a low tech approach so your mom doesn't have to deal with CO2 gizmos. You will need to pay attention to the lighting as many (if not most) tanks come with hoods with lights that really don't support plants well.
As for substrate, sand or fine gravel. Many here use sand and it's great, but you do have to rinse really, really well to remove the fines (dust like particles of sand).
(Sand does make for a denser planting medium making displacement by fish less likely).
Say hi to yer mum from yer new friends at TFK!