That depends on what you require. For me, performance is more important than anything like LED or RGB fan lighting, so if it were me I'd move the two 120mm fans that come with the case to the rear and top rear locations as exhaust, and then if I was trying to do this as cheaply as possible while still getting good quality, performance oriented fans, I'd put two of these up front as intakes.
PCPartPicker Part List
Case Fan: Noctua NF-P14s redux-1500 PWM 78.69 CFM 140 mm Fan ($14.95 @ Amazon)
Case Fan: Noctua NF-P14s redux-1500 PWM 78.69 CFM 140 mm Fan ($14.95 @ Amazon)
Total: $29.90
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2019-09-27 14:39 EDT-0400
If you want to put VERY good fans there, then I'd look at something like the Noctua NF-A14 chromax.black.swap fans.