Most likely you want the W10M native experience... Rather than wading into the Insider builds right away. Also, Lumia is going to be your best shot at native W10M and that Lumia Firmware is a huge value added package. Don't discount it. It is $ for $ the best tweaked package with the most depth in terms of Windows Phone UX.
Gently used, leftover stock or Budget seem to be the way to get in and test if you are not sure about staying.
So for new that leaves 550 or 650. Between the 2, I'd say go the extra for 650. Reason being the better SoC, upgraded cameras and double the internal storage. And the thing just looks schmexy.
Microsoft Lumia 550 vs. Microsoft Lumia 650 - GSMArena.com
Gently used/leftover stock. This gets trickier. In general, more recent is more assured for things like battery health. Also, leftover will be better. It's that unknown number of charge cycles that is the enemy here.
Lumia 1520. I have owned one since August of 2014 and it continues to be my daily. It truly is a beast of a phone. Outside of the Windows Phone community it may be a bit unknown and under rated, but it is one of the finest phones ever built, IMHO. Nokia really hit their stride on this one. It is also one of the largest phones ever built. So if size is a factor you may want to give it a pass. Because this device has been out since November of 2013 I would urge you to look at leftover as opposed to gently used. Those battery charge cycles... This is probably the only x20 series phone worth chasing and there are many compelling reasons why it remains so.
Lumia 930. Has a huge following. In many ways it is the non-XL peer of the 1520. It lacks just a couple niceties that 1520 has (no Glance or SD card). It does have the advantage of being a bit more recent (July of 2014). Battery endurance on 930 has been known to be lackluster. It's akin to iPhones in powering a rich and pixel dense display, so screen time can be a drain. It is a very nice Flagship though with a lot of style. Availability in the EU should be quite good still. Gently used or leftover should be fine with this one.
Lumia 830. Even amongst Windows Phone fans this device is discounted. The initial price point was simply too high. Good news for you, those prices have fallen. Gently used should be fairly well available and if you can find a leftover, you should look at it. It lacks a bit of punch compared to 1520/930, but it has a secret weapon. That camera is top shelf. Sure it is only 10MP... It also brings Zeiss lenses, PureView and Optical Image Stabilization. The camera on 830, just like 1520/930/950/950XL is capable of shooting .dng (Digital NeGative) which is a huge advantage in post shot editing. (It is the same thing as RAW capture.) Don't discount this one if taking photos is important to you. It is a great shooter in a wide array of conditions.
Nokia Lumia 1520 vs. Nokia Lumia 930 vs. Nokia Lumia 830 - GSMArena.com
For the x40 series we really are only looking at 640 and 640 XL. There is a 540, but IMHO you are better served with a 550 or 650. Here, I would probably say just go XL. They are virtually identical outside of screen size. The 640 XL gets an advantage in Camera and Battery. Mind you, I have a 640 and it was a wonderful bargain at $30. However, outside of the US, at the prices you'll see 640 XL should pull ahead.
Microsoft Lumia 640 LTE vs. Microsoft Lumia 640 XL LTE - GSMArena.com
So... YMMV and others will disagree... And price is always a factor... Here's what I would look at and the order I would choose them in if price and condition line up well for you.
- Lumia 1520
- Lumia 830
- Lumia 930 (because 1520 is simply more compelling in that price range)
- Lumia 650
- Lumia 640 XL
- Lumia 640
- Lumia 550
Caveat: I've only owned #'s 1/2/6, but I read a lot on the board and elsewhere...