You might be able to use an 8 speed cassette. Spec sheet says that model has a "silent clutch" rear hub, which makes it difficult to upgrade the cassette body for 9 speed.
If you want to stay with 7, you can use these microshift brifters. Definitely your cheapest course of action.
Microshift Sb-472 7 Speed Road Bike Shifter Brake Levers for Shimano & Sram | Amazon.com: Outdoor Recreation
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