You need 5 motor drivers, the latest UM2's, probably starting early 2017 miss the 5th driver. If this is the case you can try to get a board at an ultimaker reseller (very expensive) or get a chinese clone for about 75,- Be aware that you need to pay attention when using a clone board, it's been reported some have the connectors for LED's and fan's mounted up-side-down so CHECK THIS before installing.
Besides the stuff on Github, we're basically talking about two things: the second drive train (feeder, hotend, printhead parts, wiring) and some Mark2 specific parts.
Download the BOM from github. There are two sheets: a simple BOM and a detailed one. If you go for the option with the UM extrusion upgrade kit, you're most likely fine with the simple version.
By far the easiest option.
If you are starting with a UM2+, get one Extrusion Upgrade kit.If you are starting with a UM2 (non-plus) you might want to upgrade to the UM2+ at the same time, which requires two kits.
The kit includes some parts you won't need for Mark2 (shafts, but good to have these a spares, plastic printhead parts).
The extrusion upgrade currently costs € 395,- + tax.
Get a Ultimaker 2 QR extruder upgrade kit 2.85/3.0 from Bondtech.
Same here, you might want to upgrade both feeders at the same time, which requires two kits.
This kit is currently € 144,20 + tax. The Bondtech kit however only includes the feeder and the parts required to mount it, which leads us to:
Search for a UM2+ print head at sources like aliexpress.
The important parts you want to use are:
the aluminum parts,
heater and temp sensor,
fan bracket and fans.
The hotend and nozzle might be crappy, so be prepared to replace it with a decent Olsson block or 3Dsolex hotend. You should find such a print head for around $ 85. In total, option 2 should cost around € 240,- (eventually plus another € 80 for a hotend).
Of course you can use any other feeder which is suitable for the UM2. The detailed BOM lists some of them.
Magnets: You need at least 8 magnets of size 6 mm (diameter) x 3 mm (thickness), with a holding force of approx. 900 gr. It's a good idea to get some more. Metric sized magnets may be hard to source in some countries :P but we'll help you out. Then you will need some M3 screws and nuts. Find them listed in detail in the BOM.
For connecting the 2nd printhead you have two choices:
(the picture shows my home made cable stuff not the one we will have manufactured ;)
In addition the Essentials Kit and the required hardware, you will also need the following tools: The M3 hex wrench/allen key that came with your printer, or equivalent. It is the one used to adjust feeder tension, for example. A pair of pliers, a wrench or a driver set (M5.5) for loosening and tightening nuts. Some sandpaper to smooth the docking arm. A pair of pliers or vice grips can help with inserting nuts and magnets into printed parts if the holes are very tight. A hobby knife (e.g. Exacto knife) and small files are useful for cleaning up printed parts if necessary. A decent pair of tweezers is helpful for getting into tight places and precision placement.