Hi,
I've spoken to my local self-storage about just this sort of thing and they do allow cars to be stored there. However, they have some serious restrictions that largely discourage it. Many other storage facilites in this area simply won't allow any vehicles at all, for the reasons below and also because of the added fire risk - due to fuel and oil in the vehicle - and the resulting insurance costs.
This might or might not be a concern in your area, depending upon how your state licenses cars and transfers titles on abandoned vehicles. But, here in Calif. it's very expensive and time consuming to acquire an abandoned vehicle without a bill of sale and a current title. It costs around $500 to establish a new title alone, and usually takes many months. That has to be done before a self-storage business can dispose of or sell any vehicle abandoned at their facility.
So, while the local facility I use does allow vehicles, they require them to be in running condition, with current title and license, and with proof of insurance covearage.
Unfortunately, you really have to anticipate that people will sometimes just walk away from their cars and that you will have to sieze the property and auction it off to recover some of the debt owed you. My local self-storage, with about 300 units, has an auction shceduled to the last Thursday of every single month. This is typically to clear out one or two (sometimes more) abandoned units. It's a rare month that they don't have an auction.
And you might be very surprised just how large-ticket items people walk away from. The manager told me about an entire house-full of brand new, high-end furniture that got sold off in one of their auctions. It was put in storage while a multi-million $ house was being built, but something happened, the rent wasn't paid and some bidder at the auction got a great deal!
So I'd say your plan sounds like a great concept, but you will probably want to look very carefully into any hidden pitfalls. Talk to the managers of some self-storage facilities in your area for an "inside line" on possible problems that might not occur to you or us.
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p.s. Oh, and $150 a month would be a bargain, as far as I'm concerned! Here in Silicon Valley it costs about double that for a space large enough to accomodate a single car. Rents vary hugely from area to area, though. A friend of mine has a similar storage space in Medford, Oregon that costs him $80 a month.