Just wondering if the OP has considered:
1) How are you going to dish the wheels without a dishing tool? Are you going to get one? Suppose you could do it by placing the wheels in the frame, measuring how far off-centre the rim is and then adjusting accordingly;
2) If you know the model number for your hubs (like FH-5500 for an older 105 model) then you should be able to get the dimensions off the interweb somewhere and feed them into Spokecalc (an Excel file by Damon Raynards IIRC that works great, you can put in any hub dimensions and spoke count etc);
3) If not do you have a micrometer to measure the hubs? Not strictly necessary but easier and more precise than using a ruler;
4) I assume you're getting a pair of 105 hubs which is great but if you have the opportunity you could ride XT on the front or summat in 28 hole built 3x on the front - I have the latter in Ultegra, it rides great on rough roads and I weigh 14 stone;
5) I buy spokes from
Chain Reaction, good price but I can't remember if you have to get 18 or 36 at a time;
6) Sapim spokes are fine, I use DT Swiss ss db which are great too
7) Keep us posted!