Welcome to the forum I was in your situation only a few months back I guess what made my decision to go Pentax was the ability to take rechargeable batteries in some of the cameras and the weather resistance in both lens and body. I have managed to get a good collection of lenses due to what Mark said lots of good glass to last many years I have a couple of old manual lenses which aren't an issue as my pentax k-50 lets me know with a beep when in focus.
A camera is a very personal choice I think if you have a short list go try them out in hand see what feels comfortable have a play and see if the operation feels natural. If you can't decide maybe rent a couple of different ones before investing in lenses.
It doesn't matter which camera you go there will always be a bigger and better one come out the minute you have decided hehe. I haven't had an issue with 3rd party lenses but that may be because I am new and still have lots to collect
. I have pentax, sigma then samyang guess I had better get a tamron at some stage
. But no matter what camera it comes down to the foremost important thing is the person shooting
No matter which way you go you are in for a world of never ending learning and be warned there is an inherent disease that comes with a DSLR called LCA Lens Collection Addiction it takes very little time to manifest once you have a DSLR in hand hehe
Have lots of fun the folks here are great and have already helped me numerous times.