As you have already got the Cupboards installed it may be best just installing new doors and drawers.
My kitchen units sound like the same as yours I must have installed them myself about 20 odd years ago.
I have also thought about painting them but somehow I don't think that it would look very good.
I was in B&Q superstore in Northampton a few month's ago and new doors etc were reasonably priced.
Next time I am over there I will buy a couple just to see what they look like installed.
I would really prefer a new kitchen but cant really afford it.
January sales starting soon so might be a good idea to do it before the extra vat is added on the 4th.