Depends on what you would call a tune-up. No matter what bike you buy, you will need to go over all the maintenance anyways.
What is the condition? What year? SOHC?
If it hasnt been run, I would skirt some oil in the cylinders before kicking it over. Kicking it over dry is never a good idea, especially if youre slamming the E-start button burning out the starter. Take the valve cover cap screws off and get oil on the valves
You will need a new battery. You wont be able to use that old dead battery and its not really a good idea to keep it jumped to a car battery. Ive done it before, but I dont think I would do it again. XS400s depend on a good battery to start and run. You cannot start the bike batteryless, or a low battery.
Now what I would do at this point,
- Key in, power on. Oil and neutral light on? OK good. Keep the run switch OFF when not running or starting. Keeping the switch ON and engine not running will burn the coils out.
- Ground the spark plugs on the block and see if they spark when you kick it over. Spark? good. Turn the run switch off, turn key off.
- Pour some gas in both cylinders and put the plugs back in.
- Now turn the bike and and try to kickstart it. Does it start? It should fire a couple rounds.
If it fired, its good to go lol. Get it home, read the manuals and give the engine a full tune up. Clean the carbs and gas tank, get an inline filter. Do a compression test. At this point, address the rest of the bike, wheel bearings, lights, suspension, tires, brakes, hydraulic brake lines need replacement after a couple years.
If no fire, you will need to do more work. Parts are a bit pricey at times and regular maintenance is a must. You cannot bring these bikes to a shop and have them fix stuff. They wont even look it at probably, or look at you like youre nuts, or charge an insane amount.
Get photos of the bike. Photos...