Assuming this is the 1.5 engine, then the glow plugs are very easy to
change. The earlier 1.4 is much more difficult.
As far as the valve clearances are concerned, they are easy to check, the
Haynes manual tells you that one, but changing the clearance can be a little
more difficult as the camshaft has to be moved, which really means removing
the cam belt, though i did manage to do it by just undoing the bearing caps
and allowing the cam to tilt up, making sure that the belt did NOT slip off
the pulley. This allowed me to get at the shims, which are on top of the cam
followers on the 1.5. These shims then either have to be replacd with ones
which give the correct clearance, or possibly carefully ground down, but
this would not be easy. In practice, I found that I could get most of the
clearances correct by swapping the shims around, and only then had to get a
couple of new ones.
Again, the Haynes manual gives reasonable details of how to do this.
Whether you decide to do it yourself depends on your confidence in your own
ability. You should easily be able to check the clearances if you have a
feeler guage. The next bit is a little more involved.
Brian.