TM hands down, buy once cry once. You can run the TM stock ootb, don't need to upgrade until things start breaking, which I assume will be well over 5000 rounds+ on propane. If you run duster, don't expect it to break.
WE I've seen break, and go full auto cause something went hay wire, but you get what you pay for. In the end it's up to you on your budget. You'll most likely spend more on the WE trying to get it working later down the road then on the TM.
Yes it's plastic, but it doesn't look plastic. If you're a casual gamer and play once in a blue moon then I would just grab the WE, but I see retailers selling them for $230 and $275 for the WE + HK3P, I would fork out the extra little for something quality.
What do you mean "Slide lock?" You don't want the slide to lock when it shoots on empty? o.O All guns have that feature. If you mean the safety, you can remove that in less then 5 minutes.
There's a large amount of parts for the TM.. I don't think you looked hard enough so I'll spoon feed you the answers.
Slides:
Nova (Med Quality) and Detonator (High Quality), each with at least 3 different styling of slides. Costa, Nova, 10-8, etc.
Trigger:
http://www.uacofficial.com/product/m...r-for-tm-m-p9/
Hammer & Sear:
http://www.uacofficial.com/product/m...t-for-tm-m-p9/
Trigger Lever:
http://www.uacofficial.com/product/m...-for-tm-m-p-9/
BBU Housing:
http://www.uacofficial.com/product/m...g-for-tm-m-p9/
Aluminum Nozzle:
http://www.uacofficial.com/product/m...e-for-tm-m-p9/
Day & Night Sights:
http://www.uacofficial.com/product/m...y-night-sight/
Springs:
Somewhere on Ehobby