My suggestion in view of your dilemma is simple although might be difficult & inconvenient in view of travel restrictions & that is to try each model. Your car is personal to you & whilst advice is essential as you are witnessing here, there is no other way of properly evaluating what is right for your needs.
The V8 IS a joy, & will certainly be THE classic SLK but ....... there are reliability issues, as soon as you build up the mileage the value goes down, running costs MIGHT be an issue now or possibly in the future. Consider whether you REALLY will use the performance on our congested roads but of course there is the cache pride & bragging rights of having the most powerful version with the best noise & what is most important of all to many - an AMG badge!.
The V6 will give comparable performance without some of the reliability & cost issues, they are thin on the ground & it's not a V8 which might prove irksome.
The diesel will surprise you torque-wise, depress you noise-wise & thrill you running cost-wise if the latter is your choice of delight. The last time I looked the adverts. were awash with Derv jobs, most with bogo-basic specifications, so make of that what you will. Certainly the budget route to SLK ownership though.
Leaks seemed to be mainly related to pre-glass roofs about which Mercedes were well aware & according to most sufferers the manufacturer & even more their dealers couldn't care less about. DIY application of lubricant & a little adjustment generally solved matters.