I was in a similar spot to where you are now. After doing tons of research and configs, I gave up on the home-built model. I was not confident in the uptime aspect of the home-built product. Instead I went with HP P2000s (DAS, SAS) and Seagate drives (10K.5 and 15K.3s, all 2.5", whatever you do, don't buy HP drives... crazy expensive). Out of the box, they only support 4 HA servers but that can be fixed by adding HA LSI SAS switches (~$2,200 each http://www.cdw.com/shop/products/LSI-SAS6160-switch-16-ports/2266936.aspx?enkwrd=ALLPROD%3a|LSI%2520SAS6160%2520Switch|All%20Product%20Catalog).
You havn't touched on what these 300 VMs will be doing. You said performance is very important but what kind of performance are you looking for? Sequential, random, etc? I have found that smaller R5 sets work best, 8 drives. While R10/100 would be good, you are going to loose 1/2 of the space right off the bat. Plus, I think you are setting yourself up for failure. If you want to have one big array, when you have 300 VMs all trying to it hit it at once, you are going to have a que longer then the Mississippi river. Again, this is just my opinion, take it or leave it, just don't bitch at it please. I would suggest creating 6 or 9 sets of 8 drives in R5 and then spreading the VMs between them. Also, I would suggest using SAS drives instead of SATA so you can take advantage of dual ports in case of one of many things dies, controller, cable, connector, backplane, etc. With that being said, HA controllers, I assume, would be very important to you. If not, disregard. I was unable to find a good, dual controller home-built setup. Lastly, I would HIGHLY recommend 10Gb ethernet. The Dell 10Gb ethernet solutions are rather good for the price (http://www.dell.com/us/business/p/managed-10gigabit-ethernet-switches).
I know some of the stuff I recommended may be a little pricey but if you want to run 300 VMs, you need a quality setup or else reliability will suffer. Also, keep in mind that all of this stuff can be had for much cheaper then advertised price. I have an awesome rep at CDW that has been a tremendous help with my projects. Let me know if you would like his info. He is a no-BS sales guy. I hope this was helpful. If not, pose some more questions and we'll see what we can do to help.