Buying used 20hp, should I have it tuned?

If you have an excess of cash, take it to a mechanic for a tune up, lube, water pump and lower unit check.
If cash is scarce, then you'd best start learning how to repair your own.
Start by purchasing an OMC repair manual for your specific year and model of outboard.
The cost of that manual will pay for itself during your first repair,
and save you even more cash over the time you own the outboard.

 

Not much to tune-up except spark plugs. If the motor idles good and doesn't stall or ''cough'', then the carb should be good to go. Ask when last water pump, L/U oil done. You can always crack open the bottom fill screw for the L/U oil and let just a couple drops od oil out to check for water.

 

I agree with Brett, notice he said OMC manual, not just any old manual.  Some of the after market manuals are a waste of cash.  The OMC manuals often cost twice as much, but are worth every penny.  Learning to drain and clean the carb(s) can also save your butt out on the water.  If you know how to drain them, should you get any gunk in them (and eventually you will), you know it will happen at least 20 miles away from the dock.  Being able to do a quick clean/bowl drain on the water has saved me some long polling sessions and/or possibly expensive tows.  Welcome to the world of boat ownership, unless you have the cash, get used to having some grease under your fingernails. 

Swamp

ETA: that motor looks nice and clean. Depending on the model, you may be able to replace a few parts and make the motor a 25hp. Others here are more familiar with that than I am.

 

keep an eye out on ebay ive picked up two for under 25 bux each shipped one for a the omc v4s and another for a tohatsu 60b. just make sure you get and oem manual the rest of them are garbage -i got a good parts guy if you need one and i can walk you though a carb job (with pics) if need be
if its possible id try to get the current owner to do a test drive for you .... youll rule out most of your engine trouble if it gets up and goes then id go for it and when you get back check the gear oil for a milkshake this will rule out your lower unit troubles looks like a really clean motor for the age cant beat a two stoke on simplicity and easy maintenence
just put oil in the gas change the lower unit oil yearly and fog it when you dont plan on using it and itll last FOREVER

case in point -starts on the first pull EVERYTIME pushes my 13' noe 12mph loaded with gear and 6gals gas and me 175lbs

 

"i know next to nothing about outboards" suggesting a manual to help you mabey isn't the best advice - there's no substitute for experience

i'm gonna go against the flow here and give you some really good usefull advice - take the engine to the local service facility - have them check it out for you - reason being,this is what they do - clearly,they have much more experience,they will know what fails and why - when you pick it up,ask them what you can check - they should be glad to show you...

 

Kreepa, while I won't disagree with you on the principle of experience, you only get that by doing and smaller and simple 2-stoke is a very good place to start I also really agree with getting the seller to let you test the motor/boat. Offer to pay for gas/ramp/etc. Make it clear you are not just kicking tires. I don't buy anything for "real" money without a shakedown run provided I'm not buying it as a project/fixer upper anyway and the price is very good. Still, if you can run it, do so.

Swamp