KDX 200 1987 Clutch/Suspension Problems

I recently purchased the above-mentioned motorcycle. Although the motor has been fully refurbished, I have been experiencing problems with the following;
Clutch - When cold the clutch will not disengage fully i.e. after starting the engine and engaging first gear, the bike jumps forward slightly. When the engine has warmed up the problem disappears and I can even start the engine in gear with the clutch depressed.
Rear shock/suspension - If the rear of the bike is pushed down, it only partially returns to the correct position. If I pull the rear end upwards, there is approximately 2-3 inches of travel before it reaches the correct position. I know that the shock needs replacing but is this the only reason for this problem or might there also be a problem with the suspension?

The term "correct position" that I used might be a bit vague. What I meant by that is that the rear of the motorcycle does not return all the way up after weight has been placed on the seat. I know that I am making somewhat of an assumption regarding the correct position, but this is based on the fact that if I pull the rear up, it still has a bit of travel and then "locks" into position by itself.
I have already taken a look at the linkage but can't find anything that appears suspect - I have also lubed the linkage bearing surfaces.
You will have to forgive me for this question, as this is all still new to me - what do you mean by setting the sag? I see that there is what appears to be an "expansion chamber" which in turn is linked to the bottom of the shock assembly. Can/must this be pressurised? What type of maintenance can be done on the shock itself?
Why I think that it needs replacing/repair is the fact that the rear throws out at speed even on a relatively good dirt road and also the fact that I can easily push the rear fully down. My opinions might be totally incorrect.
I have browsed through many of the posts regarding both front and rear shock maintenance with regards to various oils, springs etc., but where do I start? I have spoken to a number of riders in my area but it appears that they aren't really very concerned with regards to these areas of maintenance.

Many thanks for your help and advice.
I believe that the linkage/swingarm seals, bushes and bearings might also be part of the problem, as I doubt that much attention was paid to this part of the bike. I also doubt that the shock has been inspected in a very long while.
Maybe the best bet is to take it to a dealer for inspection as suggested.