Buy the enlarger capable of the largest print you will ever want. The size of your enlarger will be determined by the amount of available space in your darkroom. 8x10, 5x7 and even 4x5 enlargers tend to take up a considerable amount of counter space AND headroom. Don't forget to take into account the height of your darkroom ceiling in purchasing an enlarger. Condenser enlargers increase contrast and any defects in the negative and image.
Most enlargers offer a condenser source but few serious printers use one.
Diffusion is found on most, if not all color heads. This of course is up to you. But remember this, it's the enlarging lens that makes or breaks a print. If you want you could buy a used enlarger and save your money for the best lens possible. Be careful with your enlarger purchase.