srs5694 has pretty much nailed it. If you can swing the acquisition of a 4x5 enlarger, do it. These big machines were built with the commercial lab in mind. Accordingly, they are very sturdy and hardly anything ever breaks on them. Any repairs that might be necessary are easy to make because of their robust construction. The only problem you might have with the operation of a big machine like that will be in making small enlargements (smaller than 5x7 prints), because it might be impossible to get the enlarging head close enough to the baseboard to do so. A longer than normal lens works in these situation. When I want to make 4x6 prints from 35mm negatives with my Omega D4, I use an 80mm lens, usually intended for use with 6x6cm. or 6x7cm., instead of the usual 50mm lens normally used for 35 mm work. For small prints from medium format negatives, I switch over to a 135 mm lens normally used for 4x6 in. negatives. It all works out perfectly.

On the question of light sources, these can be broken down into two separate major types. There are diffuse light sources that spread the light from a lamp to evenly cover the negative with soft light, and condenser type light sources that collimate and concentrate the light from a lamp with little or no diffusion. There are several subtypes of diffuse light sources. Condenser type light sources are much simpler.

Condenser lamp houses typically contain a single tungsten lamp and a set of condensers or lenses to focus the light onto the negative stage. There may or may not be a filter drawer designed to accommodate variable contrast or color printing filters. Only the very oldest and most basic enlargers do not have this feature. If you prefer this sort of light source, I urge you not to overlook this though. Working with variable contrast filters below the lens is clumsy at best and demands that you keep the filters clean and blemish free. Using filters above the lens is not nearly as fussy.

The diffusion type lamp houses come in several different varieties. All, with the exception of cold lights which use fluorescent tube lamps, use some sort of tungsten halogen lamp, a light mixing chamber, and a set of filters designed either for B&W variable contrast or color printing. Light from the lamp is directed through the filter pack, into the mixing chamber where it is bounced off reflective surfaces, and out through a sheet of opal plastic or glass. Dichro or color heads, are the most common type of diffuse light sources, and can be used for printing onto all types of B&W papers, both graded and variable contrast, and for color printing. For printing onto graded B&W papers, simply dial out all filtration. For printing onto multi-contrast or variable contrast papers (same thing, different terminology), varying amounts of magenta and yellow filtration is used. Consult the tech sheets for your paper for starting point recommendations, and note that exposures will usually need adjusting as you vary filtration. Diffusion type lamp houses designed specifically for variable contrast papers are relatively rare, and cannot be used for color printing. They do, however, attempt to minimize or eliminate exposure compensation when changing grades. I can't comment on how well this works, but I'm willing to bet that it's not perfect. Even less common are the types of lamp houses that use a combination red, green, and blue lamps or filters. This additive color correction system is contrary to subtractive color correction system that is the standard for color and variable contrast printing, and can be more difficult to manage. These additive systems are also more complicated to implement and maintain properly. I'd avoid them unless you like being an inveterate tinkerer. Cold lights using florescent tubes are best avoided too. They may offer some advantages for a few workers, but for the beginner they offer none and can introduce other problems that need to be managed.

Of the two major types of lamp houses, I personally prefer using the diffusion type outfitted with dichro filters designed for color printing. True, you may not be able to exploit the extreme ranges that your paper is capable of delivering due to limitations in the filters' design, but I haven't found that to be a consideration. If I really need a grade 00 or grade 5 to get a negative to print well, it's not a good negative and not worth wasting time and resources on. A good dichro color head will easily deliver the equivalent of grades 1 through 4 with virtually any variable contrast paper currently on the market.

The most obvious advantage offered by a diffuse light source is the suppression of small dust specs and other blemishes on the negative. Some claim that they do not deliver a print that is as sharp as can be had from an enlarger with a condenser lamp house. Well, maybe. The diffuse light source certainly suppresses edge effects or Mackie lines, and also suppresses apparent grain a little bit. I think edge effects and Mackie lines look kind of stupid on a print anyway unless the effect is used for aesthetic reasons.