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I'm not trying to be mean, but I'm going to sound that way.
I would recommend the DE10-nano kit that's used for the Mister FPGA retro system.
It's $140 on Amazon here: https://www.amazon.com/dp/B07B89YHSB/ref=cm_sw_r_cp_apa_fabc_PuV6FbQRKQ7KG
There's tons of code and help available for it.
But to be honest, I can tell you're at a much earlier level than that. I suggest you learn basic microcontrollers 1st before moving onto FPGAs.
Just a simple $20 freedom board from NXP will teach you much about basic CPUs, peripherals, and coding.
There's also the STM32 boards. Just pick an inexpensive board and follow the examples and read all the code.
At $20, you can throw it away if you learn EE isn't for you.
FPGAs are used to create, develop, debug advanced CPUs, logic, and systems. They can be difficult to write good code for, can be unforgiving and difficult to troubleshoot.
If she's into old school computers, consoles, and arcade games, you might look at the Mister project -- https://www.youtube.com/watch?v=1jb8YPXc8DA . It's based on the Terasic DE10-Nano SOC FPGA (reasonably sized FPGA + Dual Core ARM - $130 - Amazon https://www.amazon.com/Terasic-Technologies-P0496-DE10-Nano-Kit/dp/B07B89YHSB and has cores (with source code) available for everything from PacMan to the Commodore 64 to the NES (some cores require an additional external SDRAM ~ I got mine for $20 from https://www.legacypixels.com/mister/ . There are other accessory boards but they aren't really needed ) .
With Mister, it's not just an FPGA, there is a whole community she can learn from and contribute to. Useful sometimes if you don't have a specific project in mind.
I'm running it on a Spectrum Next:
A more versatile FPGA is the Mister: https://www.amazon.co.uk/Terasic-Technologies-P0496-De10-Nano-Kit/dp/B07B89YHSB
The Mister can be run as anything up to a PS1 or a 486 PC
You can get the $130 DE10-Nano through Amazon. Free & faster shipping if you're Prime. https://www.amazon.com/Terasic-Technologies-P0496-DE10-Nano-Kit/dp/B07B89YHSB
Another option is getting a DE-10 Nano FPGA board. There are FPGA cores for pretty much every computer and console up to about the mid 90s. And FPGA is not emulation, it is simulation. Basically, the hardware is recreated in the FPGA chip. This allows for clock cycle correct re-creation that emulators cannot match. The MiSTer project wiki is the best place to start reading about it - https://github.com/MiSTer-devel/Main_MiSTer/wiki
You will need a OTG USB hub and a SDRAM chip addition(can be found on eBay) at the very least. Figure about $250. More if you want to buy keycaps to match the C64 keyboard.
If you really, really want a legitimate hardware C64, you could build one yourself.
https://www.tindie.com/products/bobsbits/sixtyclone-commodore-64-replica-pcbs/ are new mainboard PCBs for C64s. You would need to source all the chips and components as well as power supply, keyboard, case, etc, but in the end, you'd have your own C64. It might prove more expensive than buying one, it might not. And if you aren't handy with a soldering iron, it is not the place to learn.
In the end, I'd say just run an emulator on your computer. If you want a joystick, get the one for The C64 with micro switches. If there's a game you absolutely must play but won't run on an emulator, then either getting/making a C64 or running an FPGA core are your only real options.
First you need a DE10 Nano board (heart of the MiSTer)... Amazon sells it as well as a few other places.
after that you decide what else you'll need or want. 128MB RAM expansion is recommended as well as an I/O board (Analogue vs Digital). USB expansion board is a bonus.. Real time clock expansion.. Etc etc... Also of you want an enclosure/case or not.
It's a bit "do it yourself" and various people supply pre-built boards for the DE10 Nano. The real name of the game is finding the best price (don't forget to add in shipping) and to find a reliable person or company with the expansion boards in stock that can ship within a reasonable amount of time.
I personally bought all my expansion boards from this guy.. Super friendly, shipped quickly and good prices too.
Also Etsy and many others... Use google, lol.
Amazon sells the main board. Digi-key has it for european customers.
There is more info on what daughterboards to get on the MiSTer github page or on the atari forums.
You can buy the main board on Amazon.
I got my add-on boards from misteraddons.
Basic information for new users is here.
USB Hub with DC Splitter + Switch and USB Bracket
PCB Fan Plate and Heatsink
USB Wifi Adapter
Fractal Design Node 202
Sheet of ABS Plastic
Panel Mount Ethernet Extension
Dual Port USB Front Panel Adapter
2x Panel Mount Dual Port USB Extension
Micro SD Extension
PC Power To AC Adapter (so I could make use of the built-in power connector of the Node 202)
PC Power Cable Inline Switch
M3 Standoff Kit
Blank I/O Shield
You can still mine Digibyte at home with a GPU. What would be the point of replacing a GPU algo with another GPU algo...? Grøstl is a less complex hashing algorithm and it performs well on GPUs and even on CPUs. We can't keep using ASICs and GPUs forever as you can see some people are concerned about the environment and forking to POS has its cons over POW. So why not be the first blockchain to implement a FPGA mining algo.
For those that are getting into mining and want a cost effective/cheap/efficient mining route, a FPGA can be bought for around $130. I just ordered mine from Amazon yesterday and will be installing it today. There are other places to buy from but I wanted to get his guy up and running ASAP.