Deathstroke's age in the comics is a bit fluid. It often depends on the creative direction of the story. Usually, he's thought to be in his mid-40s to early 50s.
Sure, Deathstroke can show up in Suicide Squad comics. It varies though, as his involvement is based on the creative direction of the writers and the needs of the plot.
You should go for 'The New 52: Deathstroke'. It presents a modern and engaging portrayal of Deathstroke. Another one is 'Teen Titans: The Judas Contract' where Deathstroke plays a significant role and it's highly entertaining.
Deathstroke has made a significant number of appearances in comics over the years. It's hard to give an exact count, but he's been a recurring villain in many storylines.