Thriller in my experience has always been well pressed. Main thing is too check condition of record if you buy used. Check for warping, scratches, and make sure it still has good grooves. I do not recommend alcohol for cleaning. Record machines are great but are expensive! I recommend using Discwasher D4 with the wood handle. Very good at cleaning and runs under $20. Audiophiles will tell you otherwise, but the mat on the rega is perfect just way you received it. Invest that money in more records! Oh yeah, no to Neil Young Greatest Hits. Just buy the complete albums! Neil Young fanboy here!

Dynamic Range Database http://dr.loudness-war.info/album/list/year?album=thriller can offer a hint at sound quality. Thriller sold a ton, there are lots of used ones out there. But buying used is all about condition. And even visually good looking vinyl will not reveal damage from a worn or miss-adjusted stylus. I bought my copy of Thriller used on a whim for $4. It was good enough for me, I was not looking for anything special. The bottom line is that if you can play the album before purchasing you will never know for sure.

If the absolute best sound quality it is important to you, then your safest bet is to buy new. The guys at Vinyl Asylum may be able to offer specific recommendations.

If you have a used record store that is local, then spend time talking to the people there. If they are a respectable business, then they should be willing offer lots of help & guidance. I am fortunate to live very close to one of best used record stores in the country http://www.jerrysrecords.com/ If i bring something home & am unhappy with i can return it.

AFA cleaning is concerned, it is absolutely critical for the best sound & preservation of your vinyl IMNSHO. Never play a record that has not been cleaned. Any contaminants that are in the grooves will be pushed into the vinyl. The force exerted by a stylus with a 2 gram load is over 2000 psi. This applies to brand new records, too. New vinyl is often dirty with residue & release agents left over by the molding process.

If you can not afford a vacuum RCM right away, the get a Spin Clean http://www.spincleanrecordwasher.com/
I use Audio Intelligent brand cleaning fluid & rinse with distilled or Aquafina water.

I use a felt mat. You might try a cork or some other, but the only way to know how your Rega will react to it is to try something.

Hope this helped.

Enjoy the wonderful sound of analog!
Bill