I own a 08 Tundra bought new, and have a lot of experience with the 4.0 V6 in the current 4Runner and previously in the Tacoma. My hunting buddy has the Tacoma with the 3.5 V6. My experience is based on this.
The Tundra has been the best truck I've ever owned. The only non-wear and tear issues I've had have been a water pump around 80K and a starter around 90K.I will say that I've done a LOT of short trips over the years, so that could be a factor in the somewhat early starter replacement. The water pump wasn't a huge deal, but the starter was like $1400 to change (!). That is a known problem with the 5.7 V8; i.e. super expensive starter replacements. Otherwise that's it. The 5.7 is a top of the line engine from Toyota, as it also goes into the $85K Land Cruiser, and is built to last. From what I see, the 3.5 has not been as reliable as the older 4.0, not to mention the 3.5 doesn't seem to have the grunt of the 4.0 (I've driven both a bunch). My buddy has a 2016, and it has been in the shop multiple times since new. It seems to run ok now, but it looks like Toyota had some issues with the 3.5 when they first came out.
If you're going to go used, I had a 1st gen Tundra with the 4.7 V8, and that was super reliable. No problems at all. The 5.7 has way more power and is much more truck, IMO. For a used Tacoma, I'd go with a 4.0 vs the 3.5.
Because Toyotas hold their value so well, for me it has been better to buy new, run them 10-15 years, maintain them well, and get good money when I sell them.
Good luck in your search.
