I have come across a older 4.5 mercury(1970's) I might buy.  It needs lower unit work.  Seals, bearings maybe.  Is this something I could do myself or something I would need to take it in to do?  If so how much would something like this cost?  Thanks

Now that I think about it since the engine is so old parts my be really tough to find and expensive when I do find them.

The first suggestion would be to get a Clymer manual or find a merc manual at someplace like Ebay.  Check ebay for parts.  There's a outboard guy close to my town that specializes in in repair of older outboards and he also has a pretty good used parts inventory.

If you have to buy any of the hard parts (gears/shafts etc) new, they will probably cost much more than an on old, small motor is worth.  Your best bet is to see if you can find a good used LU if you need to do anythng much more than resealing it.

Way2slow has the best suggestion for you. If you can find a used or re-built lower unit you will most likely be better off.

Replacing the rear seal isn't usually an issue if you take your time and with some patients. However anything past that is better off left to a pro. First off the internal guts normally take specialized tools to remove and install certain parts. There are also shims and in some units crush washers that have to be set with perfect tolerances or you will burn it up in a hurry.

Another option is just to buy a new motor

I can get the motor for around $75 that for sure needs seals and maybe the whole lower unit.  But I am not sure if its a good deal or not, not to mention I been searching pretty good for the parts I need and can't really find them.

Unless you can come up with a source and price for a good, used LU, you will probably be $75 better off walking away from it.

If water has been in the LU and left there for any length of time, most likely the whole thing is junk.  I doesn't take it long to rust the gears, bearings and just about everything else in there.