The more complete your answers the easier it will be to choose the enlarger that best suits your wants and needs. To make things easier we will go through the questions one by one and provide some possible answers. Enlargers are categorized by the size of negative they accept. There isn't any reason for you to purchase a 4x5 enlarger if all you'll ever shoot is 35mm film. 6x6 or 6x7 enlarger.
Enlargers come in either black and white print mode or color.
If you plan on moving into color printing simply purchase an enlarger that allows you to add a color-printing head later. The answer to this question may depend on your film format. Often 35mm images do not look good blown up to large sizes, say larger than 11x14. If you work in large format you may want prints 20x24 or larger. Buy the enlarger capable of the largest print you will ever want.