Im in the same boat as you and I did a whole bunch of research on getting thigh pieces while being a little chubbier.

The piece won't stretch or distort if you are loosing or gaining weight gradually... it MIGHT if you suddenly lose like 25kgs overnight or gain that much but other than in that instance it shouldn't really change. Just google it and also talk to whatever artist you are planning to go to.

As for designing your tattoo.. I have quite a few and all have been designed by either me or my artists... I usually find that the artist designs really well as they're the one who is going to do the piece, show them lots of references what you would like and what you want in it and they will draw it out for you usually. Research an artist that does stuff similar to what you want and go from there, though you can pay someone to design you a piece it might not always come out like you want if someone other than the tattooist has designed it (in my opinion but i might by wrong since i did my calf piece myself and it came out awesome and different to how the artist would usually do stuff... ) just remember to communicate with your tattoo artist; get them to tell you what they think would be better and give their opinions on it all.

Talk to me if you want any other advice, I love tattoos and I have only 5 at the moment but plan to get alot more when I have the money. I think long and hard about what i get and do lots of research.