Oil based primer isn't necessary for MDF. I've used Kilz latex primer with great results, but did use an oil based paint on the finish coat. If you give it a nice thick coat of Kilz, sand it smooth, and repeat, then spray paint will do fine. Spray paint for primer may not be the best choice for bare MDF, due to how much it can absorb. But a number of coats with a good automotive, primer with sanding in-between, will probably be ok as well. You just need to make sure that it's well sealed and smooth when you're done.