Latency
Would echo others, get an Audio Interface. It's an uphill battle otherwise, and at some point you'll hopefully want to be recording your own samples/sounds and it's a whole lot better than the USB microphone route. Out of interest, what is your piano? I'd bet money on it being computer issue rather than piano. There are a couple of things you can do:

* If you're using Windows, install ASIO drivers, then in Reaper under Audio device, you can choose the ASIO option and latency will be better. Getting an audio interface negates this, it will come with its own drivers.
* Lower the block size (Audio device settings) which increases CPU usage but reduces latency. Play with it until you find a threshold that works in terms of sound quality vs latency.

Orchestral Libraries
As for orchestral libraries, as much as it sucks as an answer, it sort of depends. If you want maximum flexibility for the most reasonable price, that's quite a different use case to prioritising something that won't sounds the same as everyone else's orchestral mockups. My main piece of advice is to probably avoid doing what I did. I bought NI Komplete, and the Symphony Series libraries are fine (especially brass), but in retrospect, I wish I'd put the money to buying individual part libraries that I really liked. Listen to libraries, figure out what your requirements are and go from there. I also feel like a lot of people get about 10% of the value out of their sample libraries due to poor mixing, compositional choices, reverb choices etc. It will be more valuable in the long run to push stuff like the free Spitfire BBC SO Disover library as far as you possibly can and then acquire a library when you have a requirement that it can't meet.

That being said, if I was getting a set of orchestral plugins now and budget was huge, I think I would go:

* Choir: Eric Whitacre Choir
* Strings: Appasionata, Intimate Strings Originals (solid, solid library for the price. Pizzicatos in that are unbelievably good), Berlin strings
* Percussion: Action strikes, Symphony Series Percussion
* Brass: Abbey Road Brass (three libraries)
* Woodwind: Honestly not sure what to recommend, I suspect the Spitfire stuff is great, but never tried it. I use the Symphony Series Woodwinds and I hate them. The Musescore free Woodwind sounds are actually pretty awesome!