Which Speaker

I certainly would not get a Marantz.  I have one for my surround system in my family room driven by Definitive Technology speakers which are satisfactory for watching TV and some movies.  I am building a higher end system for the living room eventually, but can get buy with what I have.  

I personally feel more comfortable with new.  However, this group might be very helpful in finding you some good used equipment that might be better than what you thinking about.  The big thing is to avoid making some of the same mistakes we all have made.  That's why this group is so valuable to belong to.

Hi Oliver.  Yes, do slow down a bit. You can research and figure it out in 2-3 weeks of research.

Its nice to work your way up through the tiers because it gives you better ears and more appreciation for the higher models that you might have overlooked if you just jump to the top at the start.  For instance, you get Monitor Audio Bronze and think they sound great.  So good in fact that you are willing to spend more to see how much better the Silvers are so you get Silvers and yes, they are better.  You listen to them for awhile and the upgrade bug bites again and you get Golds.  Now you’re talking!  You can appreciate the Golds, their refined sound and awesome build quality because you started with Bronze’s.
This approach though costs some $ because you will lose a little bit with each sale (even buying used and at good prices).  
Its also a good idea to try to make it to a level where you aren’t going to want to upgrade right away.  
If you are starting from scratch, you might want to look into a bluesound node 2i, an integrated amp with or without a dac and speakers (you can use the bluesound node 2i dac if you land an integrated with no dac.  Later on you can add an external dac for $200-$600 and have a nice upgrade at that point.)
So, bluesound, integrated, speakers, cables from bluejeans.  That would be my suggestion.  

Yeah I don't intend to buy anything right away and might take a couple weeks anyway if I want to save up an extra bit of money. I definitely think my preference would be to buy new, plus it would come with a warranty and I'm always a little sceptical with second hand stuff, especially as I cant go and check it out. Saying that obviously if there's a stand out deal or something I'll consider it. The first website in the links above looks like a decent place to look around. 
I agree as well it sounds like a nice idea to work my way up as well as it allowing me to better appreciate the music and equipment. At the same time I want to come in at a good level though so I get a good quality sound and value, I guess I've got to find somewhere in the middle.
For monitor audio I'm looking at the silver range, gold seems to expensive and clearly silver beats bronze. However, I'm struggling to see the difference between some of the models, the 200, 300 and 500 seem to be pretty similar except the 500 appears a fair bit bigger. Not too sure what justifies the £250 difference between each of them. I've also seen the occasional silver 6 or 10 pop up but can't find anything really about them on the internet and how they compare to the others mentioned above.
I haven't heard of bluesound before so I'll see what that is too as well as sticking to narrowing down speaker options. Every time I seem to get somewhere I find a new one recommended somewhere, spoilt for choice I guess there are worse positions to be in. The most recent to come across my computer was the Jamo C 97 ii, but again can't find much on it unlike the other speakers on my list.

Its usually larger drivers or more drivers as you work up the model line. For example, the monitor audio silver 50 is the smaller bookshelf with a 5.25" driver while the silver 100 is the larger bookshelf with an 8" driver. Try looking for pictures of the entire model line together (most manufacturers do this) and it will help with perspective as you can see the relative size difference between the speakers in that model line.

Also, if you do go down the upgrade rabbit hole as many of us have, your perspective will change regarding price and value in general. When I bought my Rega Planar 2 a couple years ago, $600 was what I considered my max budget for a turntable. Earlier this year I bought a new model Planar 8 with Apheta 3 cartridge and a Rega Aria phono stage.  All together that package retails at about 10 times the Planar 2. I've done similar things with speakers/subs/cables etc. I don't regret any of those purchases but it was a process to get there.

You may find out you are quite content with your initial purchase and never get the upgrade bug (as my pal with the yamaha/ monitor set up is) or you may go full crazy like the rest of us.  If you take care of your gear you can always trade it in (save your boxes).  Used stuff didn't start appealing to me until the prices started getting higher. On a $5000 pair of speakers you can be pretty sure they are in good shape from a good reputsble seller and the 40% discount makes the purchase easier to justify. On entry level gear I tend to agree that new is the way to go not enough benefit versus the risk.

Good luck and keep asking questions if you have them.  Lots of knowledge people here on the forum.