Mate if you can dead lift 140kg X 1 the trainer will be good warm up for you, I can only dead lift 125.8kg x 1, and it is grip strength letting me down as well, I recon I could do at least 4 or 5 at that weight if I could hold on to the bar (I am using a thick smooth bar as well though making it harder to hold on to).
I bought these years ago for martial arts training, as grip strength is fairly important when wrestling opponents.
I am looking at buying another standard sized knurled bar for dead lifts and building a lifting platform as well.
In regards to the grippers;
Trainer = 100lb to close, I can do this easily for reps.
Number 1 = 140lb to close, I can fully close it for reps as well
Number 2 = 195lb to close, I can not fully close it, about 1cm to go
Number 3 = 280lb to close, I can close it about half way
May be that will give you some idea.
Personally I would at least buy the trainer (really good for warming up) and the number 1, if you have the cash get the number 2 as well, there are in between grippers as well which were not really available back when I bought mine.
Here is a link:
Shop IronMind for Captains of Crush Hand Grippers, the gold standard of grippers for hand strength
Here are my four hanging on the home gym wall: