All those titles cost about half as much in the US. You can get LT for about $650 and the full version of AutoCAD for less than $2000. Shop around in your surrounding areas. You'd be really surprised in the pricing difference from one regional rep to another.
Like he said, LT does 2D only. Also, I have suspected that it supports less filetypes and pluggins, and it seems less stable with large drawing files. I personally think my observations are not coincidence but I have no proof.
Now, there are applications which are specific to the two tasks you mentioned. There are CAD knock-off programs that retail for under $100 and claim to do a very good job at building construction and interior design and even competing with AutoCAD as a drafting platform. I don't know if it's true and I don't remember the names. Similarly, there are drawing packages for dynamic mechanical design but they cost more.
Hey, I just had an idea. Comb the AutoDesk website. I haven't been there in about two years, but I know they've always given away 30 day trial CDROMs. That might be the way to go if you can get something. By Day 30 you should know how to use it LOL