A 1,400-hour turbocharged engine that is making metal is almost certainly due for an overhaul. That could be a good thing, as you will know how the engine has been run from day one. However, the purchase price has to reflect the fact that it's runout, rather than just getting on in life. You should be able to roll the cost of the overhaul into the loan. If you have a lender let them work out the numbers. Engine time is closely allied to aircraft value, so they don't really have a problem with that. Avionics is another matter, however, because the labor is so great the installed value is at best half the cost of buying and installing said devices. That will be tougher to borrow money for.
Does the current owner have an engine shop? The problem with having the seller do it is you're just asking for a bare minimums approach. It's your only engine. Get it done the best way you can. If the seller has a good engine shop, then make it a separate transaction from the sale of the airplane to minimize the temptation to cut corners.