You won't be able to carry your RAM over so everything can be sold to help the cause.
Something like this is what I'd probably go for:
PCPartPicker part list / Price breakdown by merchant
| Type | Item | Price |
|---|---|---|
| CPU | Intel - Core i5-8600K 3.6GHz 6-Core Processor | $279.89 @ B&H |
| CPU Cooler | CRYORIG - H7 49.0 CFM CPU Cooler | $34.89 @ OutletPC |
| Motherboard | ASRock - Z370 Extreme4 ATX LGA1151 Motherboard | $156.98 @ Newegg |
| Memory | Corsair - Vengeance LPX 16GB (2 x 8GB) DDR4-3000 Memory | $194.99 @ Amazon |
| Prices include shipping, taxes, rebates, and discounts | ||
| Total (before mail-in rebates) | $676.75 | |
| Mail-in rebates | -$10.00 | |
| Total | $666.75 | |
| Generated by PCPartPicker 2017-11-03 13:26 EDT-0400 |
You can get a beefier cooler to get right at the $700 limit.
The 8600k is probably the best gaming chip on the market right now in terms of the performance you get at the price point.