I agree with Derrell. No question (apart from the format issue, but I suggest getting more than one enlarger if you can).
The type of head hasn't been discussed much. When I had the space available at home I had four enlargers: a Chromega for colour up to 4x5 and three condenser enlargers for B&W, one for each of the three general formats (4x5, medium format to 6x9 which I used a lot, and 35 mm). That way I never had to switch condensers, I only had to adjust the lamp focus on the medium format enlarger when I switched from a 105 mm lens for 6x9 to an 80 mm lens for 6x6.
That's why I would suggest getting at least two enlargers: the Focomat for everything up to medium format and a diffusion / mixing box type of colour or multigrade enlarger for 4x5. It is no big deal to use multigrade filters above the condensers in a 35 mm or medium format enlarger (better than below-the-lens filters) but for 4x5 the convenience of a multigrade or colour head is probably worth having because the advantages of a condenser head are slight for 4x5, at least from my experience. The filter issue only matters if you will be using multigrade paper, of course.
Summary: get two enlargers if you can, but if you have to stick with one then make it the a colour or multigrade 4x5 enlarger like a Chromega.
Common lens focal lengths:
for 35 mm - either 40 mm wide angle (for larger enlargements with same head height) or 50 mm (most common)
for small end of medium format - 63 mm, 75 mm or 80 mm
for 6x9 - 105 mm
for 4x5 - 150 mm
For optimum quality you can select the lens based on enlargement. There are different lenses for different enlargement ranges. Google for data charts before selecting a lens.)
