Hi Duncan
It seems that, due to your soft heart/guilty conscience you are getting a couple of issues mixed up here.
You can, of course, offer your tenants right of first refusal. You can even offer a few helpful hints regarding how they might purchase the property. Beyond that, however, you are treading in a minefield.
Sure, they may not know that 106% loans are available at Standard Variable Rates, sure they may be interested to hear that you 'would be prepared to leave 5-10% in the deal secured by a second mortgage' .. but think for a minute:
If you were wanting to buy a property, wouldn't you want to negotiate the best deal possible? The lowest price possible? By offering to 'leave 5-10% in the deal' they may not be able to negotiate the lowest price. In fact, they may agree to pay a price which is more than valuation.
Lenders and Valuers would consider this an 'off-market' transaction, not tested by being put to the market in the usual way. This is not an arms length transaction. As you have enjoyed an amiable relationship with the tenants over time, how would they feel if the valuation came back '5-10%' less than the contract price? Wouldn't they feel that you had taken advantage of them and of the previous good relationship?
If you are offering them the right of first refusal, and thus avoiding engaging an Agent, that is already a 2.5-3.5% cost saving to you. It may be worth your while to have sworn valuations (not market appraisals) prepared for the properties to determine what would actually be fair market value.
However:
Why are you selling? If you are selling due to your change in lifestyle and drop in income, don't you owe it to yourself to get the best price possible for the properties? Why give money away? If you mix the situation up no one will emerge the winner, not you, not your tenants, and both parties may feel unhappy with the result for years to come.
You may find that the best favour you can do your tenants is to give them plenty of warning that you intend to sell the properties. Let them know which Agency will be handling the sale. Apart from that, it's up to them what they decide to do. They have to look after their own best interests, you have to look after yours.
Step back a bit, you can see more clearly from a distance.
Cheers
Kristine