Old MH's are Old MH's plain and simple. MH's manufactured before 1979 are inundated with formaldehyde, asbestos and myriad caustic and carcinogenic the only way to deal with this issue if you're really going to be investing in multiple MH's is this:
If you're going to do a remodel you may as well do it all the way. Because by the time you finish knit picking all the repairs and replacements of fixtures, utilities, appliances, cabinets, flooring, wall coverings, paint, paneling, siding, roofing, et all you might as well either remove the old one and replace it with a nice new HUD Manufactured Home.
If you're going to have to replace it with a smaller home or even a park model, if the park owner and building jurisdiction will allow it because of legal nonconforming setbacks and encroachments on the existing grandfathered one then you may find that this is your only choice.
However, that stated, if the MHP or home is not in the path of progress then it may be a big waste of time and money. Parks, infrastructures, roads, exterior fences, et. al. are getting older by the year and won't refurbish on their own.
If an MHP or resident owned MH property is not in the path of progress the values will keep deminishing as the rents increase until there comes a point when the MHP is condemned and the property rezoned for higher and better use. I've seen this happen time and time again.
So in conclusion I will say that yes, MHP's and/or resident owned MH's can be a good investment if you really take the time to do your due diligence and have the wherewithal to spend the $$$ and the staying power to hang around long enough to revisit your investment capital and then generate some ROI and appreciation.
My 3 cents/sense worth adjusted for inflation.