Also interested in the build kits folks have used.
Mine is Rival crank arms, shifter and rear mech with a 41t chainring and a bash guard from BBG. Hope Hoops 29er wheels (run tubeless with seatlant with regular Maxxis Raze tyres) with BB7 road brakes. Finishing kit was mostly from the classifieds – mid range alu stuff. Total cost was about £1,500 using a mix of good internet prices (Merlin etc), a couple of small bits from the LBS and the classifieds.
I started off with a 44t chainring upfront but have now gone to a 41t after churning away in some muddy races. Similarly, the rear mech was a SRAM X7 until I damaged that in a muddy race, and I now have a Rival one instead. Hopefully that gives you a few ideas to get started.
If you want to do it more cheaply, you could probably save some money on the wheels, get a cheaper seatpost instead of my 2nd hand Thomson and so on.