Hi, congratulations, the Magnifax 4/4a is a fine piece of equipment, maybe the best "budget" enlarger that is out there IMO. Yes, you will need the condenser no. 2 for 6x6. There are two condensers, "No. 1" for 35mm only and "No. 2" for the everything up to 6x9. Basically you can use the larger no. 2 condenser for everything from 35mm to 6x9, but not vice versa (obviously), that means, if you want to do 6x6 you have to have to the larger condenser. The 35mm condenser makes sense if the exposure times for 35mm become unfeasibly long with the larger condenser.
Nova Darkroom in the UK and Fotoimpex in Germany still sell (remaining) parts and accessories for the Magnifax, but the supply situation has become difficult, I couldn't find any condenser parts listed. If you have trouble finding a condenser for the larger formats it might be an option to search for a color head (Meopta Color 3) instead. It can be had cheap on Ebay auctions (should be under 50 Euros with power supply and at least one light mixing chamber). The color head uses four different light mixing chambers for the different formats (24x35, 6x6, 6x7 and 6x9) and these are (with the exception of the 6x9 item) still readily available new from the vendors mentioned above. Whatever you do, don't buy the Color Head 4 as it has some disadvantages compared to the model 3.
For various accessories have a look here.