Although I don't use it for beer, I have a 9 cu. ct. chest freezer that sounds like it would serve depending on the size of your carboy (I use buckets). Most of the freezer is 25 inches deep, with the area over the compressor about 19 inches. It would handle four cornies, a bucket and a CO2 bottle.
I would suggest you take the kegs, carboys, etc. that you want to fit in the unit and arrange them together, then measure the amount of space you need. You might consider using cornies for lagering rather than carboys. Lifting a glass carboy into a chest freezer without breaking it requires a lot of strength.
You can certainly use the same unit for lagering and serving, but not for fermenting the lagers. Fermentation temperatures are a little too high to preserve top quality.
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