Yes, an upgraded new version of SONAR can solve a lot of your headaches.  Cake has a upgrade Sale going on now for early SONAR X series.  Call sales and see if they can help you.
 
SONAR comes in various strengths, although the engine is the same the goodies aren't.  Think of it like a loaded car.  You pay more but you get more.  The top level of SONAR gives you a built-in drum machine (see above), built-in synths including Dimension Pro (a sample playback synth) and Rapture (same engine but the included samples are more synth-like rather than acoustic).  There is a bunch of other synths and effects and reverbs included, so you don't really need anything more than SONAR.  It is a good way to get a running start.
 
Then later you can add the Native instrument stuff, and there is cheaper ways to do it.  I bought a NI Kontakt library, Heavyocity, on sale which could use the "free" Kontakt player.  I upgraded to a paid Kontakt for like $200 during their Christmas sale.  The next summer or so I bought Reaktor for another $200.  I bought the hardware Maschine on sale which came with more synths and can now upgrade to their (un) Komplete collection for another $200 or their Ultimate for $400 (NI has 1/2 price summer and Christmas sales every year).  Like Cakewalk, they want to keep you in the family.  I don't know how much money I've saved this way but I spread the pain over 4-5 years and have NI pianos and moog and a bunch of other stuff I didn't want but now like.

Thanks SanderXpander and Bitflipper for recommending I upgrade to Platinum from Sonar 2 and telling me that its cheaper version will include Rapture, Z3ta, and Active Drums! That is good news as I was planning to buy those separately after reading the user replies from others (thanks all! Fellow musicians are friendlier than poodles!). I still haven't researched soundcards. It looks like the software will prefer Windows 7 over 10 for my new PC. But guessing the soundcard won't be up to par. Still have an old Sonar 24 bit USB card from years ago, virtually unused as my old laptop was too slow. But a new PC should easily handle that, or I may research a better slot card. I'll read up on the Sonar website for recommendations. Like you Bitflipper I am so excited to return. I've owned every Cakewalk version up to Sonar 2. My wife and I are divorcing after 28 years. I am so depressed. So I could use a creative outlet. Will go back to composing prog rock instrumentals or something! Thanks for all your help Sonarians!!!!!!

Only Platinum comes with all those synths included! Though "professional still has a decent subset". Be sure to check the feature comparison page. I don't use all Sonar stuff all the time but if you're starting from scratch, Platinum, while the most money, is a very competitive package.

I'd recommend looking at Kontakt before you look at anything else. The factory library is comprehensive and technically tics all the boxes you asked for including mellotron stuff. However its also the defacto industry standard these days so anyone else that markets samples or a sample based instrument of some sort makes it available in Kontakt format. If you get nothing else, you'll probably want to get Kontakt.
 
After that, look at Omnisphere. Its a hybrid of synth and sample player and it covers a very broad spectrum of sounds.
 
Once you've got that stuff going, give a look at the G-Force Mtron. It is as close to a mellotron as you can get.
 
The Arturia stuff is also excellent if you want models of specific synths and instruments.

Thanks all for your baker's dozen replies! I'll start with Platinum and that will keep me mighty busy for awhile and too poor to buy anything else for a while. I worry I won't be able to figure out the whole shebang - so many plugins, I'll be turning into a Borg. Are they relatively easy to learn for a pea brain like me (I'm a psychologist by trade not a NASA Hidden Figure)? Will I have time to create?

That was good advice, thanks so much I have a feeling Platinum will keep me busy for a long time. In fact I worry I won't be able to figure out all those videos plugins!