Value them at what you can sell them for for cash. Typically in most parks, that will be about $1,500 to $6,000. There is no lending market for used mobile homes. So, if the seller insists they are worth $10,000+ because you can sell them for $10,000+ on terms, it is an invalid argument. Remind him you aren't a lender by trade and remind him that the "borrowers" aren't exactly people you would desire to be financing if you were into that sort of thing.
The reality is that you will have to dispose of most of them through rent credit. We've been lucky in our parks to sell a lot of homes for cash but our luck was made when we didn't pay too much for them. We typically get $1,500-$6,000 cash and that's not far from what we paid for most of these homes when we bought the parks.
As far as the banks are concerned, you will be going local on something like this. Local banks sort of do their own thing so it's hard to say. It might come down to whether or not they like you.