All said already is good advice. You need to work out what you want to do with your motorhome, understand the issues involved with each of the parameters, then the decision is solely about what suits you - that is why there are motorhomes ranging from van conversions to Concordes and every size and layout in between.
Not much mention has been made of the significance of length. Over 6 meters you will pay more on most ferries. Last year we went to Norway, paying more for the ferry to Esgjerg and at least double (we are 7.8 metres including the bikes) on every ferry crossing the fjords going up the coast of Norway. But we wouldn't want a smaller motorhome just to save a couple of hundred quid on a special trip like that - consider what you have paid for the motorhome in the first place. With fre or cheap aires or wildcamping available in most countries, ferry cost should be viewed in perspective with the whole package if you intend to travel abroad.
Fuel consumption will be influenced as much by how modern the engine is as it will be by weight, within 500kg or so. other than taht, generally a van conversion with be better than a low-line coachbuilt which will be better than a hi-line coachbuilt, but again, it's all got be viewed as part of the package.
One final issue with weight is that you will only have a licence to drive over 3500kg if you passed your car driving test before 1996, after that and you would need to take a test to get a C1 licence.
Good luck with your search; make sure you visit lots of showrooms/shows. talk to people you meet there about layouts etc, why they like, or have decided to change theirs, then make your mind up and go for it. In relation to spending money, what you spend travelling for your research will be well invested if you end up with the van you want first time - not everyone does!!
The important thing is to understand what willbest meet your needs, plans, dreams and budgets, but remember that there will always be compromises - even if you have £250k to spend, you can't have everything - the benefiits of the layout you choose can't have the benefits of some other layouts, and whatevr you buy is a fixed size with limited space!