If I remember correctly the 86 thru 88 103's had some reaally crappy guides, in 89 they changed material so now even the 86 get's the upgraded guides.(somthing for you to think about).
On what to look for on guide or seal replacement, most notice oil consumption, lower gas milage, loss of power, rough idle, and on closer examination inconsistant compression tests, due to guides allowing the valve not to seat perfectly in it's seat.
Yes, you have to remove head to do the guides.
At 145k, you would not be waisting your money on a valve job, although if you have no symptoms...
Hope this helps,