Summer of 2018 is correct. You should be fine buying that.
You can still find iPatched units on 4.1.0 around as well that can be hacked with PegaSwitch and Caffeine but as it relies on a working web browser applet, you have to be online every time to launch the exploit from a cold boot, and if you ever connect to a network without using the custom DNS settings, you will get a supernag locking you out of the browser until you update, and thus locking you out of CFW indefinitely. Note that indefinitely does not mean forever, but who knows if another exploit will ever be discovered that either doesn't rely on the web browser (unlikely due to KASLR - but you never know) or works on the latest firmware at the time you were forced to update.
The RCM exploit is definitely preferrable as it works offline, is quicker to launch (doesn't require booting into OS only to reboot into the exploit, exploit launches in a second or two from when you insert your preferred dongle while in RCM mode), and can never be patched by a firmware update.
But do make sure that it isn't suffering from any major issues before handing over your cash as you likely won't be doing any warranty repairs on a 2017 Switch. Is it 2 years guarantee by law in Sweden as it is in most of EU? In Norway it's still 5 years on devices like this but most countries aren't as lucky.
- Most importantly check that the Switch is not bent lays flat on a table and can't be wobbled back and forth)
Even mine is a little bent but if I sent it in for repair now I would likely get a patched one back. As far as I can tell it's been that way since day 1, and I have heard of and seen videos from people who got their bent Switch replaced and the replacement came bent out of the box, it is so slight in my case though that if you don't know it's there it's hard to tell other than by doing the table test.
There are some that are far more bent than mine though and since you are buying used near you you have the luxury of checking it out in person to make sure that's not the case before you hand over your money.
A very slight bend is not likely to cause any future problems but with a severe one it would be putting repetitive stress on the internal components like the battery, the solder joints on the components, traces in the PCB and the PCB itself and repetitive stress is a big contributing factor to early failure of electronic devices (See: Apple's multiple design flaws leading to this on several of their devices - which they deny is a design flaw, but yet eventually give out of warranty repairs for, while still denying that it's a design flaw. If you know how Apple handles repairs you know they would never give out of warranty repairs for free if they didn't fuck up royally and had to do so for fear of being sued)
- Check that the JoyCons aren't suffering from the drifting issue that is so rampant. You can go to "Calibrate analog sticks" in the settings to test the sticks. If they aren't suffering from it now, they likely will be sooner or later anyway (it seems to happen from normal wear and tear), and while it's not a difficult repair to do yourself, it does cost a bit to buy original replacement sticks, so that would affect how much the console is worth IMO. The less you have to replace the sticks the better.
My cousin has now suffered from drifting 3 times. The first time he lived with it for a while eventually sending them for repair which meant he had to send in his whole console even though he has a Pro Controller so he could technically have kept playing on the tablet using the Pro Controller while the JoyCons were being repaired. That's just not how warranty repairs work here I guess. That repair lasted a few months before the same issue surfaced again. Second time I did a quick fix with some spray electronics cleaner (the kind with a thin nozzle so you can direct it into the gap underneath the rubber skirt to get it inside the stick), which lasted for maybe 2 months. He just told me he was experiencing it again and I did the quick fix again, it's obviously not gonna last but if it lasts 2 months, and having the stick officially repaired lasts like 4 months, it seems worth it to keep doing that for as long as possible so he doesn't have to send in his Switch again every 3-4 months and be without it for 1-2 weeks every time.
- Check that the screen is not scratched, now obviously some wear and tear is normal and is taken into account when it comes to price, but the dock is known to scratch the screen along the sides especially on those docks that are bent inwards. And with how crisp the Switch's screen looks it would suck to have it marred by scratches. Not necessarily a deal breaker, but something to consider before handing over your cash.
- And lastly check that it's not banned by going to the eShop. Chances are it's never been hacked but you never know, banned Switches are worth less so people do try to pull a fast one and sell them without saying that it's banned sometimes. This may or may not matter to you since you're hacking it anyway but it affects the value.
There are other issues that can occur but nothing that's really improved in later production runs as far as I can tell. The JoyCon connectivity issue is something my brother who bought his Switch just before the Smash release last year also experiences at times, so it doesn't really matter how old the Switch is for that.