If it is an IPA, then you most likely can sideload it. [guide]
Can I sideload <xyz> tweak
Maybe, it depends if the tweak requires root, and it depends if it needs PreferenceLoader. If the answer to those questions are yes, you are unlikely to be able to successfully sideload that tweak.
Can I sideload on Windows?
Yes. See the page links below.
Do I need to buy a developer account?
If you want to sideload without revocations and limitations, yes. Developer accounts costs $99, but instead third-party code signing services (which starts around $20 per device), it can be used on all your devices (current and future), as well as devices of your friends or family. If you have more than 3 devices or friends, purchasing a developer account from Apple suits you best.
You can still use a free developer account, however limitation apply such as; resigning apps every 7 days & a maximum of 3 signed apps.