Personally, I'd go with a form-ply floor. Steel is good, but also heavy when thick has a tendency to get dented if it is thin and lacks support underneath. It will also rust out if you get mud, leaf litter and moisture trapped in the corners.
The form-ply is self-supporting, can accommodate tie downs wherever you want them and can take a flogging with only minor surface damage. The only down side is that it can be slippery, but some strategically placed patches of non slip tape can help with that.
If someone wants to roll out a swag on the trailer under the tent, then it will be warmer to sleep on too.
Any surface finish is only as good as the surface you are applying it to, so any rust or loose paint will have to go.
Bed liner is good, flexible and will hide a multitude of sins, but for that reason I wouldn't use it. If anything starts to crack, then you won't be able to see the cracks until it breaks completely.
A few coats of enamel will make it look new again and if you put an ally toolbox on the front it will protect most of the trailer from stone chips from the tow vehicle anyway.
Slipper springs, they do the job. Eye to eye might be a tad better, but I don't think it will make much difference for your application. If the ones you have are doing the job and the tricked up trailer isn't going to see them overloaded, then I'd pull the old ones off, wire wheel and pressure wash them and then hit them with a few coats of chassis black. They will come up like new.
The jerrycan holders, I put them on the existing deck, either side of where the bike wheels go. The holder can help keep the wheel in place and having them off the side of the trailer is asking for them to be ripped off by a tree. Depending on what state you are in, there is something rule about carrying fuel within the confines of the vehicle. This hopefully stops it being the 1st thing hit in an accident.