Hi Chas

I would not normally recommend Reaper for beginners as it is not very user friendly, however, if you are happy with it (And used to it) then that’s fine.
One of the first things I suggest you do is to download the free Komplete Players here including the factory selections, as this will give you a number of free sounds and effects, plus, many manufactures make instrument sets that are fully compatible with the Komplete Players. (NOTE: Make sure any instrument you decide on is compatible with the player version as some require the full version)
Once you have the above setup in Reaper, you can do a search for VST instruments that meet your requirements and price (There are plenty of free ones out there) that are compatible with Komplete Players. (Everybody has their own personal favourites, so it’s just a matter of trying a few out. (Before you purchase a number of instruments check the total price, as you may find you will be better off buying a full package, (Kontakt 5 or Komplete 11 for example) as while there will be many sounds that you will not use, the sheer number available means there will still be plenty that you will)
NOTE: The initial learning curve can be a bit steep (Particularly with reaper) but once you get the hang of it, you will probably take to it like a duck to water. (You will also then understand exactly what you want, making searching for suitable instruments easier)

Hope this helps

Bill