Be aware of some possible unanticipated expense:

• Install headset & crown race
• Prep (mill) headtube & fork crown
• Cut threaded steerer tube
• Chase BB shell (+ face if external BB is used)
• Align RD hanger

Riv offers outsourced (Florida builder) Synergy/Tiagra wheelsets for $250. OC rear rim, 32h. Ideal for 25-32mm. It's one of their "budget" wheelsets.

San Marcos needs long-reach (47-57mm) brakes. If you purchase a groupset, the included (39-49mm) brakes won't fit.

Without some on-hand components, your budget sounds overly optimistic. Even Riv has stated to expect $2500 at the conservative/budget end; their specs are usually for "silver" shifters, not STI. Kalloy seatpost, Kalloy/Origin8 stem, Tektro levers & calipers, Tange/Ritchey headset, and similar choices will trim initial outlay. Also, some eBay downtube shifters, Riv "silver" shifters or current Shimano r400 8sp levers are the most affordable choices. Or strip the components off a BD bike and resell the frame, as mentioned above.

If you want a new steel frame w/relaxed geometry that accepts wider tires, you might consider a Soma ES, Salsa Casseroll or VO Rando. These would save you about $400...toward better components + STI, if you wish.