Kitchen cabinets are particularly expensive ($2,000 or so for uppers + lowers), rebuilding the walls would be another $1,500+, tub/shower/toilet, etc. probably another $1,000, carpeting (Beauflor or similar high-quality vinyl flooring would be better) would be $1,000, painting would be $1,000, other miscellaneous (electrical?) probably $1,500. So I’d guess you are looking at $8,000 to completely rebuild the interior.
That is getting to the upper limit of what we would spend to rehab any 12-wide and/or any home of any size from the 1970s or early 1980s. Much beyond that budget, and we’d rather just scrap the house, and spend $22,000 and get a good-condition used 16x80 home installed that will attract a better-quality resident with less headache than the sort of resident that would live in an older mobile home - especially a 1970s 12-wide. I know your lot is smaller, so you’d probably only have to pay $17,000 for a good-quality newer shorter home.
14x46 is the smallest 2/X they make today. Have you looked into buying a used-but-good-condition newer 2/2 or 2/1 for that lot? Can you fit a home just 6’ longer on that lot? If so, I’d seriously consider spending around $17k for a 14x46 or 16x46 used home in good condition (that is an all-in price to buy and set and skirt the home). You’ll get a better resident, more rent, and your property will be more financeable with the bank - and all for ‘only’ $9,000 more.
My 2 cents worth,