This will be a short article - you're welcome.
The answer is yes, the iPad is certainly worth buying.
Why You Should Purchase an iPad
I've seen plenty of guys that are knocking and just using their iPhone to sign up their customers. If they're lucky, they are ripping an iPhone Pro Max, so it at least looks a little bit more legit. A lot of them are just ripping the iPhone SE though.
Unfortunately, they are kind of shooting themselves in the foot. A large part of door to door sales is looking legitimate and authoritative. If you look like a joker that is just who is just wheeling and dealing, people are a lot less likely to purchase whatever product you are selling. Closing a deal and putting credit card numbers into an iPhone does that. I don't know why iPads don't feel the same, but they simply feel nicer and more professional. Especially if you have a legit looking case on them.
This is why the iPad is well worth buying. When you are selling items door to door, your commission should be decent enough that anywhere from 1 to 4 sales will pay back your iPad. The added legitimacy and iPad brings will likely help you close at least 1 to 4 more sales over your summer.
On long days when I was knocking (Memorial Day, 4th of July, random Saturdays), my iPad would frequently run out of battery. When it did, I had to use my iPhone to put in the deal details. I would put my iPhone on my iPad so it looked like I was using the iPad. People just have been trained that paying via iPad is normal and natural. Meanwhile, if you're plugging their credit card details into your personal device (no matter the fact it is the same app and perfectly legitimate), people are going to start getting antsy and wondering if they really need your product that badly.
Which iPad?
Now, any iPad is good enough. I've used both the iPad and the iPad Mini and I know a couple of guys who ripped an iPad Pro (the 13 inch one - pretty dang big) and a lot of people who have used an iPad Air.
I personally prefer the iPad Mini. If I'm carrying it around all day, I really prefer as small of a device as possible - while still looking professional. However, any iPad will do the job.
Does the Case Matter?
It does as a matter of fact. If you pull up with an iPad with a ridiculous looking case, you just lost all the legitimacy you had gained from carrying an iPad around. I'd choose a simple, solid-colored case personally. Also, if you're willing to look a little ridiculous, an iPad with a strap is SUPER NICE. 8 hours into knocking, it is super nice to not have to carry your iPad from house to house.
The End
Hope this was helpful! Enjoy your knocking!