What no one has mentioned is the ability of the caravan to take a payload. Around 2012 the definition of payload changed (in my opinion not for the better), so if you are buying used make sure that you understand how payload is calculated. Pre to 2012 the two weights of interest were the ex works weight which basically defined the van as it left the makers, the later definition allows the makers to include an allowance for some items in the definition of Mass In Running Order, which is higher than ex works. See the CC and C&CC sites for the definitions. Going back to basics I like a payload of 250kg based on ex works weight. The one thing you should not do is exceed the MTPLM Maximum Technical Permissible Laden Mass. You would be surprised how much weight your ordinary kit takes out of the payload. Examples being motor mover 32-38kg, battery 20-25kg, aquaroll/pump 5kg, levels/chocks/griptracks 6kg etc and where do the food, kids, and dogs things go? Oh and don't forget the awning, groundsheet, pegs etc.
Admittedly not all of my payload will go into the caravan as i try to keep the van as light as possible. But one reason that you now see more caravaners using roof boxes is that their vans don't have sufficient payload. I see this as retrograde as having a roof box can sometimes prevent entry into some nice car parks where the owners don't want motorhomes.
Some companies will let you upgrade the MTPLM if the caravan chassis/axle has sufficient margin, but this may also need you to upgrade the tyres too. Company upgrades are paper exercises and cost around £60 Bailey will size upgrades to any of their caravans that have the chassis capacity irrespective of age. the Orion 430/4 cannot be upgraded as its MTPLM is within 3kg of the chassis design, but Pursuit 430/4 can have an upgrade to 1300MTPLM (nearly 70kg). But recently I was looking at an Eldiss van and contacted Eldiss about an upgrade, but as it was just over 2 years old they don't offer an upgrade. Recently I have ben told by salesmen that the battery is included in the MRO, a mover won't affect your payload as its very close to the axle, and most makers offer a payload upgrade service. Some will even try and convince you that your car can tow more than is ideal. I use the Dutch matching site which is superior to the UK ones which tend to be very coarse. You can tune the Dutch matching site even if the car and caravan aren't exactly as described in the lists. And it looks at hills starts, stability , driving in mountains, M-ways. It takes account of a particular engines torque, power, rev characteristics, and you can shift load between car and van to see the effects. It's in English too. However the results need to be considered carefully against your own knowledge and experience, and by talking to other caravanners.. Like all things caravanning its not like buying and driving car, you cannot test drive the chosen outfit. The site is "towing.info". For these reasons I would strongly advise that you sign up to one of the CC/CC&C courses that includes far more than just towing.
Some continental caravans (even diddy Eribas, Freedoms and Triganos) have very good payloads as their upgrades may have been installed in the works and involve higher rated chassis and axles. The new Caravelair Antares 455 in France have range of MTPLM from 1200-1500kg. whereas the UK importer is offering a free upgrade from 1200kg to 1300kg.
Good hunting.