A boxer of that vintage that has sat unprotected will need the following;
1- new tires.
2- New fuel filter and maybe a pump.
3- New DOT4 brake/clutch fluid.
4- All new oils (engine, transmission, final drive).
5- New seat covering.
6- New plastic light housings (tail lamp, turn sigs).
7- New fuel lines (the in tank lines are high $).
8- Exterminator (there will be rodent nests in and on the bike).
9- New air filter.
That model has linked ABS brakes. The hydro unit may require repair from non-use, a $300.00 cost. A new ABS unit is $2000.00.
So, total up the costs and you have in the neighborhood of $1000.00 for just these parts. As mentioned, if the switches and other hardware on the bars have been exposed to the elements, you're looking at X2 that amount...and that doesn't include labor.
The tank liner may be compromised, another expense to have it cleaned and repaired.
You're now very close to the value of the bike in repairs. I would pay no more than a few hundred for the bike.
If you are going to do the work, get JVB's DVDs. Also, I would invest in the factory REProm, another couple hundred $s.
Let us know how you proceed. There are plenty of experts here who can help.