For oscillators I considered the Make Noise DPO, Mutable Instruments Braids, and Erica Synths new Black VCO and expander. I chose the Erica Synths modules because they were the least expensive. The DPO is the most expensive. Braids is a close second to the Erica Synths modules. All are excellent in my opinion.
There are many good output modules. I chose Make Noise Rosie simply because I am familiar with it and like it. I rely on it and it delivers.
Make Noise MATHS is so versitle you just have to have it around. It's an envelope generator. It's a sound source. It's an ADSR when used with the Disting module as a gate source. It's a positive or negative voltage source.
The Expert Sleepers Disting is useful in its own right, but it is also a great learning module to experiment with functions that might cost a lot more $$$ if purchased as individual modules.
Erica Synths Black Hole DSP is not strickly necessary, but it brings a lot of fun into modular synthesis. Without it it is kind of like shooting black and white photographs when you really want to be shooting color.
Make Noise Pressure Points is purely optional but can really help when learning.
I THINK IT IS IMPORTANT to point out that there are more cost effective ways to get involved in synthesis if you don't know anything about it and putting a system together. For example, Moog Mother-32 or Make Noise 0-Coast. There is also the Erica Synths Pico system. If $$$ is no object, then take a look at Macbeth Studio Systems. They have a stand alone synth which is pretty sweet and classical. I think Doepfer and Pittsburgh Modular also make stand alone packages. Finally, there is also the DIY synth voice from Erica Synths if you might enjoy building your own synth and then playing it. And it sounds pretty good.