Could i develop an ARKit app with my Windows PC (and Unity 3D) or should i buy a Mac?

With Unity3D you have the ability to develop applications for IOS using various SDK's. This is why i am asking if we could use ARKit plugin for Unity in WIndows OS??

>have the ability to develop applications for IOS using various SDK's

ARKit is an API, not an SDK.

>use ARKit plugin

ARKit is an API, not a plugin.

Questions on 3rd party tools/Windows are best put to their related support structures. Forums here are primarily focused on Apple's IDE.

You need a Mac to build and debug iOS/Xcode projects. If Xamarin has support for the latest iOS SDK you might be able to do most work in Windows although you still need a Mac to build. However, I wouldn't recommend Xamarin for this type of hardware specific development at all. Save yourself some time.

You may be able to use Unity in Windows to do part of the development of assets and models.