Honestly I've found the process of buying a supergun a little confusing.
The overwhelming consensus seems to be: Buy a HAS, do not buy anything else other than a Minigun or maybe a MAK. But I've also seen lots of recommendations for Sentinel, Windy, etc.
Unfortunately, Windy don't exist anymore, Sentinel is out of production, Minigun is being made in tiny batches by seemingly random people (especially if you're a newbie like me, who doesn't recognize the bigger names in the scene yet, so you won't know if it's been made by some rando or a reputable person) and the MAK.
So, best I can tell right now the only options are
Retroelectronik
Minigun (if you build it yourself, or lucky enough to get a pre-built)
MAK
A custom from someone like JNX who I hadn't heard of until this morning.
Build your own.
More than happy to be corrected if I'm wrong and there are more available. ebay seems to have a lot of no-name ones that look and sound pretty dodgy and ship out of china, I'm not counting those. If you trust those, you can hop on ebay and get a supergun for as low as like 10 euros. But I wouldn't be comfortable putting a 10 euro no-name piece of equipment near the OSSC I bought just last week.
I decided to go with a Retroelectronik Progamer, and then immediately started to doubt that as I researched the thing a little deeper. I am pretty concerned about the RGB and Audio output not being properly attenuated as was brought up by RGB, is mentioned in a fair few threads, twitter arguments, and a few youtube videos. I'm not an electronics engineer, but I can totally believe that a malfunction could result in a fried OSSC or a GScart if those voltages are sent directly across. I don't have a GScart, but I do intend to use this stuff with a brand new OSSC. So now I'm frankly too scared to even plug the thing, unfortunately I bought it through their website and not ebay, so I don't know if it will be easy to return.
So I hopped back on ebay and bought the only pre-built Minigun I could find in the UK. I got lucky from the sounds of it. Only problem is, now that I don't trust the Retroelectronik DB15 converters (pinout is different to neogeo and undamned, but in general I just don't trust it, so I'll only be using the converters with the retroelectronik, on a CRT, keeping it well away from my OSSC) so looking into controllers proved to be confusing as well for someone who doesn't have stuff laying around from the NeoGeo era.
Undamned's DB15 converter seems to have been out of stock for a while, which seems to leave the only option right now to be buying something like a Mayflash stick and gutting it and wiring it to DB15 yourself... Or maybe a Genesis pad or buying a NeoGeo stick is easier, I dunno, I need 6-buttons and will only use a stick though as I play a lot of Capcom fighting games. So there might be cheaper options that I didn't find.
Only other option I could find was Brook's NeoGeo converter. However, this seems to have pretty negative reviews compared to his other products, which surprised me. And I can't find anywhere if it supports 6 buttons, doesn't seem like it does it's not really designed for supergun but for NeoGeo. So, I bought one anyway just to have a quick way of using a controller. It might be a good option if you want 4-button though, as it sounds like a firmware update solved some issues with not saving button remapping.
I opted to grab some DB15 connectors and buy a Mayflash 500 to gut when I have time later on, but it didn't feel like a good option, felt expensive and isn't an option if you can't do a bit of soldering.
Looking for the 8-pin mini-din stuff was a little confusing as well, it only seems to be 'standard' if you're very familiar with the framemeister, so I don't honestly know why it's the chosen connector for the Minigun (I'm not particularly knowledgeable, so that's not necessarily a criticism, there may be very good reasons, but it made finding adapters very difficult). I bought the scart to mini-din cable designed for the framemeister from RetroGamingCables and there doesn't seem to be any circuitry in it so I'm hoping it will work in reverse (I haven't plugged it in yet, so if anybody knows for sure, I'd appreciate the info)
So, yeah that was my experience buying a supergun right now, May of 2020 as a newbie. I've owned a JAMMA cab for a few years, but I've never dabbled in superguns until I wanted to capture video recently, and didn't want to mess about with direct capture from my Sega NNC.
I would readily have dropped whatever the price is for a HAS (I literally don't even know how much it costs, there's no shop and pre-orders are closed) but it wasn't an option and I have no idea when it will be. I'll wait for one, while using the Minigun in the meantime, but it does seem like options are pretty limited right now for newbies, especially if you can't solder or feel confident re-wiring a few cables/connectors.
The other thing I will note, is if the Minigun seller I got mine from lists more on UK ebay, he states it 'comes with a power supply' but this wasn't true, it came with a 6-pin connector that splits out into wires you need to attach to screw terminals on a JAMMA power supply. So I had to order a JAMMA power supply as well. Not a problem, just, order both at the same time if you go that route to save you waiting. It also doesn't look like the one in that twitter thread, it's very very similar but without the number display (I assume for voltage). It was sold as Advanced Minigun but, I don't know for sure if it is, or what version. Board just says 'minigun supergun' and 'advanced edition' on the underside and it has a THS7374 sitting in the middle, it's shorter than the one in that twitter pic too. I'm guessing I have the V1
edit:
I'm actually considering buying the 2.5 off ebay, does anybody know if the 2.5 is significantly different other than the voltmeter? Or am I good with the one I have?