If I hadn't read any Flash Gordon at all, I would start with the website I linked above that has the last 20 years of Sunday strips available to read for free from King Features that owns it, just to get a sense of whether I liked it or enjoyed reading a continued adventure strip in Sundays format before I plunked down any cash on it. If I liked it, I would look for the easiest available collection of the Raymond strips. Maybe check out your local library system and their inter-library loan department to see if you can get copies of the collections to read without investing money in them to see if you like them first, and if you do like them and want to own them, then you can look at going about getting the edition of the collections you like best. Check to see if they have the Dark Horse Archive editions of the collected comic books too. Libraries are great ways to sample comics you aren't familiar with without risking a lot of money on it, then you can spend the money on stuff you know you will like and want to own permanently. I know the Ohiop public libraries had the Checkers editions of the Raymond stuff available on interlibrary loan when I first looked for them about a decade ago (before the Titan or IDW editions came out) and after I read them I started tracking them down.
My intro to Flash was the Gold Key comics in the 70s, some Al Williamson paperback collections I got at a Scholastic book fair and the Filmation Animated series (followed by the Buster Crabbe serials which aired on a PBS station a couple years later which my dad was a big fan of. I came to the Raymond originals much later, but they became my favorite version of Flash.
But everyone has their own manner of approaching things and trying things. Some people refuse to use the library, some refuse to read collections wanting only original issues, some refuse to read digitally, etc. It all depends on your preferences, your resources, and the availability of the material in the formats you want.
-M