If you have an internal or external DVD drive, purchasing a used PC, Xbox 360, or PS2 disc is the most reliable way to own the game legally.
You can often find physical copies or "offline mode" digital deliveries through retailers like Amazon and eBay.
If you have an Xbox, buy the old disc. If you are on PC, wait for an EA sale on NFS Heat and mod it. Only download the 2005 ISO if you have a modern antivirus and a willingness to tinker with Windows Registry files.