I have a chance to purchase a used Colt SP1. Would this receiver make a good donor for semi auto M4? Is there anything I should look out for on the receiver? Thanks  

It's OK if you get a good deal I guess.    Most people seem to want a lower with the raised fence around the mag release and small mil spec front pivot pin so they can install a push pin.    The colors will probably differ a good bit in the lower and upper if that bothers you.

CH444,  No this would make a great rifle to M-4.

The one thing that you will want to do is just swap the barrel, fore arm, and butt stock.  This will leave you with a A-1 upper w/o a forward assist. You will not have to worry about using a off-set pivot pin, nor the upper/ lower not color matching if you use the SP-1 upper.

The high points to this is the A-2 sight is a waste on the M-4, and most people never use the forward assist anyway.

The parts needed to convert are as follows:

Car Carrier-stripped(use your old bolt and key).
M-4 buttstock with buffer and buffer spring.
M-4 barrel with gas tube and Hand guards.

Note: You will need to file the new front sight(pin), due to the A-2 sight housing being a little higher, but only a few strokes will be needed to zero the rifle.

Hope this helps.