1. The reason why the Hürkuş is being looked at (to replace the T-37) is lower flying costs than the K-8. Yes, the cost will be higher initially when the new plane is bought, but the savings from fuel cost over a period of 15-20+ years will make up for it.
2. The Hürkuş is very different from the Mushshak. The Hürkuş is turboprop powered with performance relatively close to an old jet like T-37 but with much lower flying costs, and the Mushshak is a piston trainer for basic training.
3. Pakistan already has experience and infrastructure for the Hürkuş' engine - the Pratt & Whitney Canada PT6 via the Cessna Grand Caravan EX, King Air 350ER, etc. PAC began working on a MRO site for the PT6 in 2015. So the engine maintenance, which makes up a big portion of servicing, is already in place.
4. If the Hürkuş is bought it won't replace K-8, but take the role of the T-37 before K-8. Pilots from the Hürkuş will move to the K-8. Why not go straight from Mushshak? That will be costly in long-term operating costs, it might be cheaper to have Hürkuş in the middle to provide most training (and more flying hours) before sending to K-8.