My standard advice for anyone looking to "upgrade" to Windows 10: make damn sure your boot drive is an SSD. Trust me, on a HDD, Win 10 has a nasty tendency to slow down to a crawl over time. It doesn't matter which type of SSD you have, be it SATA or NVMe. As long as it's from a brand you've sorta heard of, it's probably good enough. I'd say 240 GB is the minimum you really want; with 120, you have to do some heavy shuffling of files to avoid filling it up. Depending on your budget (which I'd guess is "not much"), I'd go for a 480 GB SSD if you're willing to spend extra.
I cannot stress this enough. Get an SSD if you don't already have one, though make sure it's compatible with what you have. Those 2.5 inch SSDs are the safest bet; most desktops can take those as a drop-in replacement.
You can keep the old hard drive as a storage drive. Just don't install the OS onto it.
BTW, Windows 7 product keys work with Windows 10. Just write down your product key somewhere, then enter it during the install if it asks for it. I assume you know where to get Windows 10, but if not,
here's Microsoft's official tool to upgrade to Win 10 or make a pendrive that has Win 10 on it. My understanding is that upgrading straight away lets you skip the product key entry part, since it just reads the current product key and gives you a digital license for Win 10. Then again, if you are replacing a HDD with an SSD, it's probably best to copy all the stuff you care about somewhere, do a clean install on the SSD, then put it all back onto your clean install of Win 10.