If you look in Trip Reports, I put one up mid last year after a visit. I loved the place so much I went back again later in the year!
Your questions are very open ended and all depend on budget. There are a couple of agencies who specialise that I came across - Boutique Sri Lanka who are UK based, ExoticLanka Holidays who are AU and I see Jacada Travel now do Sri Lanka. If budget is not the motivator then I highly recommend those guys!
They have itineraries -
https://www.jacadatravel.com/indian-subcontinent/sri-lanka/ as do the others which you could use as research.
Key for your trip will be getting a driver for your time there to cover the ground you need. There is an article on trip advisor about how they price driver hire.
I did not make it to Udawalawe Park but that's apparently best for elephants.
I did visit Yala and to be honest I did not enjoy the experience of safari drives there - certainly do not expect the animal numbers you'd see in Africa, nor expect the same consideration to conservation. Keeps tear around the park to sightings and then crowd around one animal.
Accomodation - how long is a piece of string! We stayed at Resplendent Ceylon properties and rate them highly, stayed at a couple in the Cinnamon group which were fine and stayed at Kandy House which was lovely.
Timing - I went smack bang in monsoon season in October and found the weather fine, just had a little bit of rain. Our main trip was March and weather was fine - Yala was hot - over 40. There is good information online about time to visit.