The Line-X is pretty expensive, for my purposes, it was the only choice.
You should be able to find a cheap bed-liner for ~$110, be sure to get an under-rail style if you're getting a tonneau cover. I'd spend the $110 on a bedliner, not a cargo mat. Most car parts places carry the bedliners, difficult to order one on the net because of shipping difficulties. Call around & get some price quotes to find the best price. A top of the line drop in bedliner is ~$170.
The bed extender looks good, I'm tempted to get one myself, but may settle for a cargo net, much cheaper. In my case, Ford has licensed the technology & the specific models for Fords are available only through Ford, I can buy a generic one from the manufacturer, but it doesn't have the same features as the Ford-sourced bed extender, because of the licensing by ford.
Bed covers: If security is an issue, best to go with a hard tonneau cover, they lock & are difficult to break into, the rolling style uses up some bed space, while the fabric style require storage space elsewhere if you take it off.
Definitely buy everything from someone other than your car dealership. Dealers make big $ on these items.
Once you settle on what you want, do a google search & find the cheapest prices.
For tonneau covers I found
This site had the cheapest prices on the Extang Cargo Max I just ordered.
The V6 in the Tacoma is the best choice, it's a real hot rod engine & should make a significant difference in your driving experience.
Installation is simple, the manufacturers spend lots of money ensuring accessories can be installed easily, don't be intimidated. @ the most, you may need a screwdriver, wrench or socket set, & maybe a drill.
I don't know anything about alarms, but the one in the link looks really cool.