I've built more small block Fords than you can shake a stick at and done a handful of big block Fords.
Cost depends on condition of your core, what you want out of it, and the quality you want to put into the build. If the motor is just down on power/leaks oil/whatever like a typical old engine you could get away with a simple cleanup and re-ring, lap the valves by hand, etc. and spend a couple hundred bucks tops. Big thing if you go that route is to check the taper of the bores, if taper is too much (.006" is really pushing it) you will wear out your new rings in a season or two and be burning oil again, also valves and seats must be in reasonably decent condition.
A basic mild rebuilt, if pinching pennies, will run around $400-$600 in machine work to hot tank, bore the cyls, valve job, turn crank, depending on what needs to be done and if any other work is desired.
Then your parts to build the engine, basic kits for a mild build are around $300-$500 depending on waht you get. $500 will get you a nice kit with forged pistons (such as:
http://www.summitracing.com/parts/FEM-CSMHP801-311/). Throw in a few other components, double roller timing chain ($50), maybe cam+lifters ($200) if you want a little more performance or your orig cam is worn excessively (Comp makes some good marine grinds. Misc BS items ($100-$200). If you want more performance a little cleanup of the heads wouldn't hurt. Mild port job on the exhaust side just to clean it up. Basically a free 10-20 HP if you take the time to do it.
So basically if it's a typical worn engine and you want to do it right you can look at around $1500 between parts and machine shop costs for a mild build with good components. Small blocks are cheap and easy. Use marine parts where appropriate (brass freeze plugs, oil pan, pickup, etc.), but I wouldn't worry about marine intake and head gaskets unles the boat will live in salt.
As for using a used engine... I wouldn't replace a Ford engine with a Chevy, but aside from that if the used engine will fit in easily and is only a few hundred bucks then go for it if you want to save some money. Before putting it in I'd check compression and bearings to be sure it's in good condition. I put a used SBC in my boat. I didn't feel like taking the time or money to build a new engine mid-season, just wanted to get back on the water quick and cheap. The SBC came out of a car with 20,000 miles on it. Ran great in the car, compression checked out and bearings looked good. It hasn't given me any trouble and only cost $300. I've put 40+ hours on it this season without a hiccup. Just gotta check it before dropping it in and replace any automotive parts with marine, because there's no walking home when you break down on the water.