I like the advice already presented. Here are few more thoughts for this and future dates.
1. Think of this as dating. Seems kind of obvious but it needs to be said. Do what you would if this was a "real" date. Manscaping - recommended. Shave, light cologne, nails trimmed, nice clothes appropriate for the restaurant - yes. Offer to open doors for her, etc. - yes. Be charming and confident, never condescending or rude - yes. IOW, do not treat her like a hooker. Sex may be (almost) guaranteed, but don't act like it is.
2. $500 is probably in range for Metro DC, but I'd say it's on the high side. In general, her 1st offer is going to be buffered up in expectation of you negotiating down to a mutually-agreeable level. Just remember that how you negotiate is critical. My recommendation: Always come from a position about your budget. Never negotiate about what she is "worth." She is "worth" the sun, moon and stars. Just ask her! But your budget may only allow you to give her x dollars y times a month.
3. If you don't mind her knowing where you live, and you don't think your neighbors will raise their eyebrows when you start bringing home random 20-ish babes a few times a month (or week!) then go for it. I do take these precautions when a new SB is coming to my home for the 1st time.
A. Lock all my doors and windows. I lock the door again once she is inside.
B. Hide my wallet, spare cash, and car keys in separate locations before she arrives.
C. I keep her allowance in an envelope in an easy-to-reach but not obvious location near my door. When she's ready to leave, I walk her to the door and hand her the envelope.
D. Have beverages ready to serve - both booze (wine, beer, etc.) and soft drinks/water in sealed bottles. She my be nervous about being drugged, raped, etc. So a sealed bottle of water may be a good choice.
E. Don't be in a hurry to get naked. Go slow until she sends the vibe.
Please let us know how it all turns out.
Life is good
The Cat