Yay, welcome to the wonderful world of strap-ons! It sounds like the number one thing, honest communication, you and your gf have down on lock, so you all are already off to a great start 
So, we have a really great guide by Carly that is perfect for pegging 101! This answers a ton of questions and has product suggestions too, but I’ll also throw my personal favorites below as well.
For hemorrhoids and just a really great all around soothing balm, check out Southern Butters Backdoor Balm. They are a really great line and we talk more about the balm here, it’s a great thing to have on hand for backdoor play. I don’t personally have experience with hemorrhoids, but anal play with them is possible, just have to prep and watch after. There are some decent guides online with really good tips!
To start, I would look at getting a multiple toy set that is perfect for starting out smaller and working up. Blush has a really great affordable one. I like these sets for beginners because you have options, and you can mix and match while you get into your groove with anal play and seeing what size you like. I personally prefer a dildo over a plug, just a different feeling/sensation that my partner prefers, and I feel like a dildo you have more to work with, especially for strap-on play. And if using hands, make sure to wear a glove and lube up well, even a tiny hangnail can be a big discomfort. As a person with a vagina, I personally stick to toys only when it comes to anal play with my partner, just because it’s safer than my hand and also easier for clean-up, but that’s just my personal preference. I also like to use a dildo without the strap and then use a wand massager at the base to add some vibration as well. Cheaper than buying a vibrating toy, and just adds a different sensation, especially when you are starting out and finding out what toys and sensations you prefer.
Carly mentions it in her article above, but I’ll echo my love of Sportsheets strap-ons! They are affordable, come in different styles and have a wide size range. I’m plus size and femme, so my favorite is this one with the corset detail. I’m a size 26/28, and it fits me with room to spare. I wear it a little high on me, but it’s always been easy to adjust and comfortable, and I like the cut with the corset how it sits at the top of my butt, it’s super cute!
There are a ton of different lube options, but my go-to personally is Sliquid Sassy. I like that it’s water based so it’s more compatible with a wide range of toys, and I’ve always found it good for both vaginal and anal play.
Main thing like I said earlier is just communicate and listen to your bodies. Take things slow to start, take your time and try different things to see what feels good for the both of you. Being curious, having fun, and listening to each other is the best way to dive into strap-on play!