Welcome to the forum.
First things first. What's the intended use of the boat - purely leisure trips up the coast on a calm day, or are you looking for something with a relatively deep v hull that will handle the rough stuff?
Think about seating. Leisure seating such as bench seats and console seats are fine for calm seas with the family, but as soon as the wind gets up, in-line jockey seats with wrap-round backrests and grab handles would be the way to go.
5m ribs are a good size, small enough to fit in most 18' garages, and easy enough to launch and recover single handedly. Always more fun with more people - but you get the point.
Then there's budget. You can easily spend £15-£20k for something new with a modern 4-stroke engine, A-frame, chart-plotter, VHF, etc, or look at something second-hand?
Back to your questions.
Buy from big manufacturers?
There are added advantages, in that there's a decent dealer network and after sales service. Yes Zodiac or Avon would be desirable, however it depends on the specific model of boat, condition and engine combination.
Buy a rib from China or lesser known make?
Definitely taking a gamble. Look at what's on the market here. A dealer will struggle to resell it if there's nothing to compare it against. These ribs are fine if there's demand for them otherwise you're going to take a bigger financial hit at p/x.
Any views on Mako ribs?
No idea, never heard of them. Regards Mil/Pro division, the Avon and Zodiac (same company now) are world leaders in these types of hypalon boats, but expect to pay top dollar, and not worth it if you're planning to sell in a couple of years.
Should I bother what it's made from?
Yes and no. Zodiac leisure ribs are made from Strongan Duotex (PVC). Finish is immaculate, as they're produced in a part automated factory using heat-welding techniques, glued transoms, etc. Fine for everyday leisure use, but will degrade over time. Hypalon on the other hand is stronger and you can expect at least 15-20 years use before thinking about retubing. It's a manual operation to build and glue. Incidentally Zodiac are now producing some of their Pro range in hypalon.
Buying trailer separately?
You get what you pay for in my book. A 5m rib will easily sit on a unbraked 750kg roller trailer. Top manufacturers include Indespension, De Graff, Rapide, etc. All need to be set up properly so the boat sits well. Annual maintenance should include bearings, periodic maintenance or prior to towing - grease tow hitch, check tyre condition and pressure, etc.
Anyone know of a good 5m rib that fits the criteria?
The $66m dollar question. Everyone has their own preference, myself included. Here are a couple, although I'm not rating them in any particular order.
Zodiac 12 man Pro
Second hand Zodiac boats - Western Marine
Ribcraft - FAFB