Here are a couple of good reference sites for ISO/Conex container systems and related:
Sea Box:
Sea Box
Scandinavian Shelter Systems:
3S Group
This is a composite image of one of the SSS expandable modules:
I think the aesthetics of an ISO-based system are what one makes of them. There is no need for something like this to be ugly, even if one were to start with a used container. It is possible to select a used container based on it's age, appearance and quality, which will affect the price.
Using the engineering principles of the ISO container, it is also possible to build your own using alternative materials, or take a hybrid approach by using a commercially available open-frame ISO plan and handling the flooring, walls and ceilings as required for your particular application.
In my vicinity, based on proximity to the Port of Houston a high-quality, relatively rust-free twenty-foot container can be had used for about $800 - $1300. A single twenty-foot standard container has a TARE (empty weight) of about 5400 lbs.
I am budgeting an upper limit of $150K on the trailer, containers, materials, appliances, etc. exclusive of the HDT, which I'll select after making more progress on the trailer/modules themselves. I will likely use a Volvo 780 or T2000 with an integrated sleeper, but that selection will be based on the weight of the trailer rig, availability and pricing considerations nearer the time I complete this project.
I am convinced this is a highly practical, economical way to end up with a very high-quality, livable, versatile rig. I am interested in maintainability, reliability and doing as much of the work including engineering on my own. There are now a wealth of materials, tools, etc. that make this a pretty straight-forward possibility for someone with reasonable skills. And I think in the end the cosmetic and aesthetic result will be positive, as well as unique in the RV domain. I don't foresee being rejected in an RV resort based on the rig's envisioned appearance. I've discussed that with numbers of folks including the management of the places I currently visit, and don't believe I'll be dealing with turn-downs if I decide to visit an RV resort...