Oil leak

I'd be more concerned with the water leak than the oil leak.
Older motor, just a little leak on the top crank seal not a huge deal.
Water leak could be big bad news...

It could be a simple fix, maybe just tightening the bolts, but I'd get it fixed if it is more than a drop. It will get worse over time...
 
The oil thing is pretty standard on older 2 stroke motors, I wouldn't get too concerned if it's not a lot of oil...

Look on the bright side, a new 150 is probably only about $16K...
 
Seriously though, you are way ahead finding those problems now than a month from now...

Be careful. They hide bolts on those dam things. You'll have to disconnect the water tube and take all the bolts out, look under the anode there's usually one there somewhere. When reinstalling the lower unit make sure the shift shaft engages... Other than that it's a piece of cake.

As Iwishtwofish stated, youtube is your friend on how to's.  Helped me with my first time carb rebuild on my 15 hp Johnson.
 
Also, lots of helpful people on this site:
http://www.marineengine.com/boat-forum/forum.php

If you have decent mechanical skills, you shouldn't have a problem changing the water pump yourself. I would highly recommend getting your hands on a shop manual however. Perhaps you can find the necessary documentation on-line.
 
Going off memory. First, remove the prop. Next, remove the trim tab (that anode with a fin that's on the underside of the cavitation plate). On that motor, I think the shift linkage connects just below the power head. If you take the engine shroud off, look for the top of the linkage (I'm guessing starboard side). There'll horizontal rod that runs parallel to the transom, and the linkage will come up onto the motor area from the mid-section. There'll be a clip that holds the two together. Again, I'm kind of guessing about how the linkage connects, going by past OMC motors I have owned. Some smaller ones, you have to drop the lower unit to disconnect the shift linkage.
 
Once you have the linkage disconnected, remove the lower unit. There should be a bolt up under the trim tab.
 
When I change an impeller, I ussually buy the whole kit and replace the housing and the housing seal. You don't want to do the work only to find out the seal needed replaced as well. I have also seen worn housing liners causing the pump to not push enough water.
 
Remove the water pump housing, impeller, and key. If you bought the kit, it probably comes with a new bottom plate. If so, replace that, the key, impeller, housing and seal. Oh yeah, before you put the seal on, put a little molybendum lube on the lip of the seal. Always do that with seals. I always use a torque wrench when re-assembling. Yeah, I'm pretty anal about the work I do.
 
Check the seals on the exhaust housing and the water tube. Re-install the lower unit. I use Indian Head gasket compound as a thread locker. If you use a different thread locker, make sure it's not one that attacks aluminum.  
 
Check for fishing line around the prop shaft seal and grease the prop shaft before you re-install the prop. Re-connect the shift linkage.
 
Test run on the muffs.
 
Good luck. I hope you get her running.
 
Is that motor oil injected, or pre-mix?

eyeassassin

Injection

 
Check the line that runs from the oil tank to the pump, if it's the vinyl line, replace it. The vinyl line's been known to split. Make sure the oil tank contains nothing but oil. Sometimes they get water and other contaminents in them. Check all those plastic wratchet clamps on the hoses. They get brittle and break when they get old.
 
I believe '86 was the 1st year for the VRO. They had problems with VROs the first couple years. But if your's is a VRO and still going strong, either you've been one of the lucky ones, or the pump has been replaced. Alot of guys would disconnect the VRO and run pre-mix just to be safe.