How do I know if the condenser is included?
The condenser is housed in a plastic box that is inserted above the negative holder. If there isn't a condenser there will be a big gap just below the filter holder (which is in the main M670BW head).
You should receive two condensers. The smaller one (Siriocon 50) is only suitable for 35mm; the larger (Siriocon 80) one can do 35 and 120 but will result in longer exposure times for 35mm than the smaller condenser. The smaller condenser is unsuitable for 120.
The M670 is a nice enlarger. I have a 670 and M601. The 670 is slightly more plasticcy in places, but they both have similar column mechanisms. You need to make sure the column holds properly and that there are no missing teeth in the rack (which is on the column) and that the pinion gears are in good condition. They are plastic so can be damaged if misused. The head is held in place with friction pads and needs occasional adjusting to ensure it's solid and smooth.
I don't really use my 601 but the 670 is my main enlarger. The recommended 150W bulbs are starting to get tricky to find though.
For the price, I'd expect to receive the enlarger with both condensers , negative carrier (UNINEG) with masks (SIXMA/SIVOPAR pair 35/66/67 etc.) for at least one format of 6x6 and 35mm and preferably the ANR glass (SIVOLGA AN). For 6x6 for example I use one SIXMA 66 mask and the SIVOLGA AN glass on top in the UNINEG negative carrier. For 35mm I tend to use the Sivoma 35 and Sixma 35 mask pair.
You also need the correct lens mounts SIRIOPLA 39 for 35mm and up to 6x6 or if you want to enlarge 6x7 with a 100/105mm lens you'll need a VEGATUB 39.
The M670 is modular and you can also swap out the BW head for a colour head which uses diffusers rather than condensers (Again there are two sizes and you should ensure you get both unless you only use one format).
