Hi Mate,
I did this recently, Its a really fun little project and the gains are pretty big.
As the Daily Fail is always a good web benchmark, i've seen some tests that report that the Homepage is 50mb without Pihole, and like 2mb with....
That saves time, makes the internet feel quicker and comes at no loss to you. Also frees up the internet for other stuff etc.
Anyway to answer your question.
I have a 330Mb connection and was quite anal about speed and generally impacting that so rather than worry about it I just went for the Raspberry Pi3B+ (fastest one, for me the slower ones aren't cheap enough to be worth it)
There is a well known kit available from rainforest called "Ucreate" ,you want the "Official" power adaptor and this one comes with it, also all the basics you need including a OS on an SD already for you to use.
I'd recommend that.
Then after that I would follow this guide:
https://pi-hole.net/
https://github.com/pi-hole/pi-hole/#one-step-automated-install
It's pretty easy, the whole thing should take about 1hr from booting the Pi to installing PiHole
Other things to consider:
Make sure you either reserve the DHCP address the Pi gets from the router or ideally set a static IP for it outside of your DHCP range.
Change the DNS address of the DHCP pool so all devices are protected from the ad's and tracking features. Other wise you will need to change manually
Ideally, wire the PI into the router rather than use WiFi to reduce latency
Finally, if you want the Pi to auto boot and run Pihole when there's a powercut etc then you need to do some further work, I haven't done this yet but am told this is easy.