There can, in theory be a land-based military. As you accurately noted, the outer space treaty forbids such - and even if we was to not sign it, the founding ethic of "a demiliterised free base of scientific knowlege in space" suggests it not happen regardless. I do agree that having land without an active military presence is possibly unwise, but also think placing an Asgardian armed force onto the surface of the Earth isn't a particularly good start with aims of promoting long term peace. It's a lot harder to have a war with nothing to fight. Land itself, as previously mentioned, is most unfefasible, at least in the short term. Either buying or building something of suitable capacity for everyone, then setting things up for it to sensibly operate and then maintaining the operation of said will be most crippling. Even if you would entertain the notions of some others and doing a similar thing but on a much smaller scale - only accounting for 200 of our 170k population, and assuredly to scale to larger operating costs - would still be restrictive in the setup of infrastructures and long term continued operation of - Which could effectively be placed into more productive ventures. Someone elsewhere put forwards the ideas of floating structures parked possibly in international waters. That could easily and feasibly be scalable to a few thousand I'd wager. If designed modular and to spread the load across the total structure then it might be able to cope with a few tens of thousands of citizens. Else "small floating towns" can exist in clusters. It'd certanly give an ideal environment for testing certain technologies and principles prior to relying on them in space - like long term biostasis experiments, checking people can exist in close quarters wtihout murdering each other, etc. The largest concerns for such installations IMHO would be regular supplies, interwebs, and piracy. I doubt most nations would be eager to invade, and international diplomacy can arrange for "coverage" from several navies.

The headache with that would be actually acquiring the land. I think it's reasonably safe to eliminate a charitable donation from the list of possibilities. To find somewhere willing to sell something that would be suitable for purpose I think to be most unlikley, but if it does happen it's certainly not cheap. Possibly cheaper to construct our own island in international waters - and that's not going to be cheap either, rediculous cost attached to building even a small island - especially in deep water. Just finding this money will be a headache. Then there's the cost to the construction of the "city", the raw materials and the labour. The planning behind the supporting infrastructure and the cost of it's execution. Building a city is one thing, but if you want that to work, you want that to last, there's a lot more you need to do - a lot of it first. It's not quite just building one building next to another. All in the end figure is something difficult to make happen even utilising several methods to make happen - within any acceptable timeframe(ie: less than ten generations, assuming the requirement to build land to build on). It wouldn't be the most productive use of our time or resources. Not right now. Also, having borders with easy access at this stage is incredibly difficult to be defending. Internal to another countries borders just effectively creates a security risk for both parties.