Submariner Serti Dial HELP!

[Ardnic]

Hello Community!

This is my first post in this forum. I am fairly new to the world of watches and have fallen in love with the submariner serti dail. I now got an offer for 6000€ from a PRIVATE seller, not a trusted dealer. However, I do not have any knowledge about this type of watch and hope you could give me some help here:
So my first questions regard the risk of counterfeiting:
-What simple visual checks can I do to ensure myself that the watch is original (Character differences, serial/refernce number, hands, other details on the dial and oder parts)?!
-Can I check whether the dial was inserted afterwards, so it was not a Serti Dial in the first place and in case this is true by how much would this lower the value of the watch?!

I believe I could bring the watch to a rolex dealer to get it checked for being original before I buy it. Can I trust the result of this check and how much does it roughly cost?

Also, i am interested in your opinion regarding the increase of value of this type of watch over the next few years. Do you believe this is an "investment" where one at least does not loose money?

Again, thank you very much for your help and I hope that my questions do not exceed your patience ;).

Best Regards,
Ardnic