Mercruiser and OMC used Chevy and Ford BLOCKS or in the case of the OMC used it's own 2 stroke V4 and a Buick V8 in it's own gull wing hull, back when tri-hulls were popular. Insides of the blocks are different and all are of marine quality, made for continuous WOT operation, unlike automotive applications. Parts are expensive so be prepared for that.
The outdrive has 2 potential issues: corrosion of alum parts if stored in the water, or not flushed properly after salt water usage, and water intrusion into the lower unit gear housing. Those parts are expensive. A quick guess is easy enough. Bring along a large screwdriver with you on your inspection. With the outdrive vertical, like when you are running on the water, carefully remove the drain plug located on the bottom of the gear housing adjacent to the skeg. If water comes out first, or if the drain screw has a magnet inserted in it and the magnet is heavily laden with metallic particles (some are normal....reason for the magnet), or if the oil looks like chocolate milk, you can expect problems. If none of the above, you ought to be good to go. The constant velocity joint where the outdrive is attached to the boat mounting bracket may be worn causing vibration in turns and the exhaust hose between the boat and the outdrive normally requires replacement, especially if sitting for a long time.
Get a manual and check out the engine and outdrive servicing requirements including a new water pump/impeller.
On the trailer, #1 is usually wheel bearing problems due to water intrusion. Expect to have to buy replacement parts but they are inexpensive. Having something like Bearing Buddies, or a grease flushing axle design are big aids in keeping your bearings running right. Use a water resistant grease made for trailer wheel bearings. One is white Lubriplate and the other is Mystic in the blue-green color. Surely there are others popular in your area....read the label.
On the boat, if glass, major problems are water intrusion into wood backing members like the floor (deck), stringers (floor mounts to them) and the transom. All are repairable and not all that expensive, but highly time consuming. Other problems are plastic hulls sometimes crack (only 2 mfgrs I know of) and glass sometimes fails in light weight hulls especially where a lot of flexing is possible.....deck to hull interface and transom to hull interface.
$1000 sounds like a reasonable price even with anticipated problems that surely will occur.
Good luck
