Good tenants, living in DUMPS... let them stay?

[Hollis Cook]

I'm looking at a property that is 3 single family houses and 1 efficiency on 2 acres. (Seller was asking $369k with no offers, will accept (verbally) $340k and will refund $6k in closing costs and $15k for repairs, $319k to seller, $21k to me, 10%down--$34k). There are two nice houses that need minor repairs, currently owner occupied, estimated to bring in $1,100 (2/2) and $1,300/month (4/2). There is a single family (small, old, 1947 1/1) that is currently rented for $650, but the tenant has trashed it, has a motorcycle INSIDE, which the owners have allowed, needs a total interior rehab, estimate is $10k, but... he is a long-term paying tenant and wants to stay. My thought is why kick out a paying tenant, when I can defer the rehab cost and it's already trashed anyway. I would only do exterior repairs, estimated at $5k (Estimate for interior and exterior rehab is $15k). Much the same, there is a very small efficiency, total rehab inside and out needed. Long-term, good paying tenant wants to stay at $450/month. $10k total rehab estimate, $5k exterior/$5k exterior, I would plan to do exterior only, for $5k estimate and leave her in there. My questions: Is it wise to only do exterior rehabs and leave the good-paying tenants. I could kick them out, do the full rehab and end up with a bad tenant(s) and have to rehab again anyway. If those two tenants are willing to live in dumps, why should I care? I plan on selling in four years, for all I know, they'll still be there and I'll never have to do the interiors. How do I calculate in that I may have another $15k in interior rehab expenses coming down the line? They've been there for years, won't be able to find rentals at the price their paying in this market, and they both seem to be happy living in dumps. What are my potential downfalls? Thanks!