I understand your predicament, budget-wise, and I can tell you have done quite a bit of searching for NSTs. Fair enough, let's look at using two MOTs:
Your diagram appears correct, except that (2) is connected to the core of (A) internally, and (3) is connected to the core of (B) internally already. You will need to connect the cores together, and then connect them to ground to get a center-grounded transformer with two hot secondaries that are 180 degrees out of phase (notice the phasing dots on the transformer cores):
As you can see, on each secondary you have only around 2KV with respect to ground. However, due to the phasing, when one secondary is +2kV, the other secondary is at -2KV, which gives you 4KV between them.
Now, there isn't really a way to tell the phasing just by looking at the transformers, so you may need to try it one way, and if you don't get a very good output then you'll want to reverse the connections on one of the primaries and try again.
As for the ballast, just connect a third MOT as I suggested earlier by shorting the secondary and wiring the primary in series with the 230V HOT line.
4kV should be enough for a Tesla coil, considering the very high current. You will need to extinguish the spark gap somehow, whether you use a rotary spark gap or if you blow air through a static gap. If you don't ensure the spark is extinguished, you will get very poor performance.
And finally, I'm sure I don't need to tell you this, but I feel I must post it for anyone else who may read this thread in the future and attempt this on their own:
WARNING: Microwave oven transformers are EXTREMELY DANGEROUS and will kill you instantly. There are NO SECOND CHANCES if you accidentally come in contact with any live parts of the circuit. Make sure you take ALL necessary precautions to protect yourself, family/friends, pets, and property. Make sure you have a fire extinguisher nearby, and ALWAYS have someone watching your back when working with these transformers, preferably someone who knows CPR. Using MOTs is STRONGLY DISCOURAGED for many reasons, so only use them if you are left with absolutely no other option.