Hey buddy,
Your options for reasonably priced & up-to-date laptops with extendable GPU slots are:-
Lenovo Thinkpad T530
Lenovo Thinkpad W540
Lenovo Thinkpad SS430 (only the models with core i5 & i7 processors)
Lenovo Thinkpad TS430 (only the models without NVidia GPUs)
HP ZBook 14
HP ZBook 17...