Really creative. I thoroughly enjoyed the mixes. I have no useful advice on plugins. For me, I pick kinda randomly. I know a certain set of plugins pretty well and I tend to use them a lot.
 
One that I regularly use is MeldaProductions M-AutoAlign. It's useful for calculating offsets for mics and reducing phasing issues. It's kind cool. I know where the mics were in the room, and it often exactly calculates the number of feet between the mics (or fairly close anyway) as shown in the milliseconds of offset.

I think if you absolutely must spend some money on plugs, Ozone is a good buy. It's downpriced a lot right now because version 6 is coming out. It's a well-respected home mastering solution and will add a little sparkle to your stuff (which I agree can also be improved with regular EQing). Be careful with the presets, they're often stronger than what you need.

That said, I still feel weird giving plugin advice when you don't really know what you need ;)

I think Ozone is a good buy as regards "Bang for buck". I also like a lot of the PSP stuff as well, but Ozone has it "all in one place". They have a demo available that'll give you an idea of it before splashing out for it. 

So, I listened to the DEMO tracks and this reminded me when DEMO's were recorded back in the day.  Today, with so many tools available, frequently it is hard to tell a DEMO track from a FINAL MIX.
 
But the minimalist approach to those mixes leaves a lot of imagination up to a producer for arranging (I know there were a few spots where you already "pre arranged" some parts). Which, to me, is a good thing.
 
I do assume the purpose of these tracks is to sell your SONGS and not YOURSELF as a musician based on your OP.
 
I thought that some of your ideas were real good (albeit, couldn't understand all the lyrics too well). So, for DEMO purposes, I think both tracks do the job.
 
Good work, keep it up. No real advice from me on what plugins to recommend.... if you could be more specific of what type of processing you think you are lacking, I am sure that you would/could get tons of advice on specific function plugins.
 
Jim

Ozone 6 .............coming soon ;)
 

wavearts power suite for your bread-and-butters, if not already mentioned, demos available http://wavearts.com/

I was in a similar situation with the OP a year ago and after my searching, I went with the Christmas deal on iZotope's "Studio Bundle" (Nectar 2 (vocals), Alloy 2 (mixing), Ozone 5 (mastering)). In 15 years, this was the first time I had purchased a non-Cakewalk plugin. To reiterate comments made above, I think the best feature of these is that they are essentially "stand-alone effect chains," which fits well with Beepster's minimalist comment (and is a personal pursuit of mine as well). I had done a bit of research on options prior to this purchase, and made full use of their 10-day trial prior to buying them. Interestingly, they have features in them that are not spelled out in their manual, and I found by randomly clicking around the UIs; but the plugins themselves are very capable and tweakable, and allow for saving as presets to enable faster workflows with each use. Ironically, there was a thread about FabFilter Pro-Q shortly after I made my purchase and I saw a tutorial on mid/side EQ... at that time I thought "Oh well, I have already committed to iZotope," but turned out that iZotope plugins have this capability as well. Overall, I think these carry very high "bang for the buck" value with them.
 
As an aside, iZotope also has free guides posted on their website. These are written at a more "global" level, so can be applied to any similar plugin, but of course the details are focused to their specific products. The Alloy 2 guide has nice mixing tips, and the Ozone 5 guides have mastering advice and tips on dithering. https://www.izotope.com/en/support/support-resources/guides/

I guess I should point out at this stage that if the conversation goes to much towards 3rd party software then it should really be in the Software forum.  That is what it's for after all...
 
Just saying.

Karyn
I guess I should point out at this stage that if the conversation goes to much towards 3rd party software then it should really be in the Software forum.  That is what it's for after all...
 
Just saying.

given the thread title, how could it not be?

And Sonar X3 is also software ;)