Do you have a solid bulkhead ?
I strap a 400L upright to mine, no worries
baffles - cut sections of 80 mm diameter plastic perforated land drainage hose, 25 m length is about £40 or so on ebay delivered.
Don't protectakote ( I have but wouldn't again ) instead if the ply is dry underneath, seal the edges and fiberglass with a roof kit, 5 sq m is about £70, will wear better, look neater and will mean the floor is flat and easy to dry and keep clean, not so with a bare metal floor that is corrugated - it will collect pockets of water and dirt .
System - why fix what isn't broke ? simple is as simple does to quote forests mum
Yes an slx would be worth upgrading to, but buy longer than you anticipate currently needing, as you say you are expanding and there is nothing worse than finding you cannot reach and then needing to go down the extension route, you can always remove a section or two.
Brushes - thats a personal choice. I've two supremes, mixed medium hybrid with pencil and fan jets fitted ( swap between depending on what suits ) regular work, and soft flocked with fan jets for georgian, leaded, upvc, shop signs etc.
Everybody has different opinions, these are just mine, hope they have been of help
