Thanks, all!
I appreciate the feedback that mileage isn't a primary concern. Especially considering that even doing a road trip with a cheapie gas guzzler would probably be less expensive than other ways I like to spend my free time and money (notably international travel).
Re: associated costs, I suppose I didn't do a good job explaining that I was already thinking of licensing and insurance... good reminders, though. Worth pulling together a fuller cost-benefit analysis to look at that in more detail.
Parking shouldn't be a problem -- street parking is an option where I live in Seattle. I think if I had to pay for parking, it would be a non-starter. <snark> I could just burn my money instead, and that would at least have some consumption utility. </snark>
My gut instinct is that if I do buy a vehicle with the goal of using it as an adventure-mobile, I'll want a high clearance one. Most trails are available with low clearance vehicles, but my partner and I have both experienced a number of forest service roads and the like with low clearance and high clearance vehicles and one is way more pleasant than the other. (Not that you need 4WD. That seems to be overkill.) Plus, it sort of defeats the purpose of having 100% ownership of my own vehicle, but then be limited in my ability to have those adventures if there's snow or a rough road to the trailhead or whatever.
That said, I will definitely look into a carshare. I don't love the idea of renting cars -- it just racks up money so quickly, and you lose out on the ability to get up and go. Plus the transaction costs of having to pick a car to rent, go to pick it up, etc. There's an argument to be made for paying a little more to own your own car and get the convenience of it. Time also matters.
A carshare might be a good compromise between all of those things! And it has the added bonus of being less of a temptation to drive all over creation just because I can, rather than sucking it up and biking like I currently do (and like doing, despite certain inconveniences).
Will look into this more!
Here's a follow-up question: assuming that the van in question is in good shape, how much of a good deal does this sound like?
If it'll be hard to find another one for that cheap, then I'm a little more inclined to rush this decision-making process. If I have a good chance of finding a similar deal after I get back from my big trip, then it would probably behoove me to hold off for now, see how things shape up when I get back (living situations, how I spend my weekends, whether my other partner is correct when he says that his Outback is too short to sleep in, etc.), and go from there.
The little research I've done so far does show similar cars to be in the $7-10k range rather that $3.5-4k, and my brother says this is a good deal. But I'm not much of a car person, as mentioned, and my brother is not a Mustachian, so I don't have a good sense of direction here (pun intended?).