Put an SSD in your laptop. You won't be sorry. If you need more storage, look at a USB auxiliary disk or a NAS.
The good news is that from Windows 7 on, Microsoft got their shit together and have only one install media (per windows version) so you no longer need an OEM specific install disk. If you can still read the license key on the Windows sticker on your laptop (it will probably be underneath the battery), you can download the appropriate ISO image from Microsoft (home, pro, 32 bit , 64 bit, etc), burn it to a DVD (or use the USB creator) and install.
After you install, Windows 7 will probably not have all the drivers it needs for your system. You can download the appropriate drivers from Dell and install (if your wifi adapter driver is missing, you'll have to download the driver for it using another PC and transfer it using a USB stick).
Or, you can go here:
https://www.microsoft.com/en-us/accessibility/windows10upgrade and upgrade to Windows 10 for free*, which will most likely have all the drivers built in.
*Although Microsoft has officially ended the free WIndows 10 upgrade program, they still offer a free upgrade if you use Windows assistive technologies. You don't have to show that you in fact use them.