You could do it a bit cheaper, get a surplus pack from CTD or elsewhere ($70 or so) and have GB or Bill Springfield convert it and C/P.From a budget standpoint, Your best bet will likely to buy a barreled upper from BMP ($650), complete RCM bolt group from HKParts ($500) and a semi trigger pack and furniture from RobertRTG ($300).
Since you are gonna SBR it later, wait for later and pick up a Choate or something, by then you will certainly have the coin.
Be patient on Sturm and you might find a genuine HK BCG for $400-500. I saw one on the Parts section here recently for about $400...
(OF course Dave / DJF will do you right if you go that route.)