Interesting, I thought it was an eight speed shifter from the photo. My error.
I have several crank pullers, I think most of the ones out there are equally good. Around home I use a Pedros one. I need to pull the crank arms off one of my bikes when I pack it for airline travel, for that I use an XLC that is small for travel. I have an old Sugino one somewhere in the bin. My point is that there are plenty of good ones out there, but I would avoid the really cheap ones. You need one for square taper crank, if you do not see the phrase square taper, either ask or move on to another seller, there are pullers for other cranks that you do not want.
This is the XLC one that I use for travel, but Amazon now says it is no longer available.
https://www.amazon.com/XLC-Cotterles.../dp/B000NU2WAS
I have a couple different cassette pullers too, you can't go wrong with a Park for Shimano/Sram cassettes. I have one of those and a Nashbar one too.
You likely will want a chain whip too for removing the cassette. There are work arounds for avoiding that, but in the end if you are doing your own maintenance, you will want one. A work around is at this link, but this work around might not work if your lock ring is on really tight.
Chain Whip for Travel
Eventually you will want a pedal wrench, that is a 15mm open end wrench but it is thin, likely thinner than a general use open end wrench. Note that some pedals use an allen wrench instead but most use a 15mm open end. I mentioned above that I use a XLC crank puller for travel, that uses a 15mm pedal wrench.
You might want some cone wrenches if you want to re-grease your hubs. Size, I do not know.
A lot of this stuff can be learned from youtube videos. The Park tool ones are especially good.
https://www.youtube.com/channel/UCza...uZN-I8_XT6AH8g
I usually suggest that people look at at least three you tube videos when they study up on something, as at least one of those will have incorrect information and the other two are likely right. But, you can be assured that if Park says it, you do not need to look at other videos to verify it.
If you try to pull your pedals off, one of them is left hand thread, the other normal thread. You will figure out which is which. Bottom bracket, one side is left hand thread too.
If you have a torque wrench, use it when installing a crank arm on the bottom bracket. Google search should tell you what torque to use. I do not think you need to buy a torque wrench just for that, but you have to make sure you get them tight enough. When at home, I use a torque wrench, but when traveling and assembling my bike outside of a hostel in some foreign country, I just wing it without a torque wrench.
I have no clue where to buy a seven speed cassette, but I am sure Google or some other search tool will help. My errand bike uses a seven speed, but teh rest of my bikes are eight or more. I usually buy eight speed cassettes on Amazon, same with chains.