Are there any free upgrades available?
Sure, if you pirate. If you mean legally, no. Unless you purchased a new computer running Vista after June 26, 2009 then you can download a free upgrade.
If you buy it, where is it cheapest?
If you are a student or know a student you can get a cheaper upgrade copy here.
All that is really needed is an email address ending in .edu. If not Amazon has single license upgrades for $107.67.
And if you buy it, how many machines can you install it on? I would hate to have to buy it for each computer and laptop we have. $$$
You can install a copy of Windows on 1 computer technically unless you buy a family pack, then you get 3. The family pack might not still be available but I am not sure.
We currently have Vista. No problems.
If you are happy with Vista you don't need to upgrade.