MQDs do not include taxes and many fees, so for domestic flights you probably are looking at about $4,500 spending for 3,750 MQDs. MQDs can also be earned by spending on a Delta AmEx card ($10/MQD for Reserve; $20/MQD for Platinum). MQDs are awarded when flights are flown, so flights during the match period will count even if they were purchased before. However, flights flown after the match period but purchased during the match period do not count.
Also remember that "To be eligible for the Delta Status Match offer, a SkyMiles Member must (a) be enrolled in the SkyMiles Program for at least 30 days prior to enrolling for the Status Match offer and (b) within the last 3 years as a SkyMiles Member, have flown on at least 1 Delta-marketed flight (with Member’s SkyMiles account number included on the reservation) on a fare class that is not Delta Main Basic (Basic Economy). If your Status Match Challenge request is accepted, you may only enroll in this offer once every three (3) years."