I've said it before, but that is one fine example of an early Liner.
Since this trailer came to you out of the blue (rather than representing a life long dream come true), and you're even thinking about selling it, I tend to think that's what you're most likely going to end up doing. Since you own it for only the cost of hauling it home, selling it as is would probably yield more $ profit than trying to fix it up to some degree, and flip it.
Option 2 will just have it tying up space beside your house while you tinker with it until pursuing option 1. Too much casual tinkering might even devalue what at this moment is somewhat of a time capsule.
Option 3 as you know is the road to the money pit. It would probably cost you $2-3K just to get it polished.
From what I've observed over the past few years, I would think it is worth between $8-12K as it sits. Just my guess. I would tend to think that the ideal buyer would be a person that specifically wants to restore an early Liner as an act of love (where you value your time at about 50 cents per hour) with the intention of keeping it forever.
There is presently one for sale on ebay that is not nearly as nice as yours. It's been listed for a while for $10K+, but I'd be very surprised if it sells for that. I know someone that recently bought what he describes as the world's nicest original Liner for $7-8K. However, he is an experienced hunter/collector with a lot of trailers, and a lot of contacts.
On the other end of the spectrum, I can recall seeing a really nicely restored (frame off and fully polished) '49 or '50 sell for $18K a year or two ago.
Good luck if you decide to sell, be patient, and don't give it away.