Inheriting fish is always a problem.
Don't worry, we'll get you though this and you'll soon have a tank to be proud of
With an internal, you are definitely overstocked, and you have other problems too, but first get the tank sorted. Though if you can find a shop that will take the plec off your hands it will make things much easier. Too many common plecs are sold, and these fish get massive - well over a foot long! And if you could find a new home for the crab you'd also be doing the fish a favour.
Unfortunately, whoever said the pH is what it is meant to be isn't being very helpful as pH is what it is when it arrives in your tank - there is no 'what it is meant to be'. It can be anything between 6.5 and 9.5, the legal limits in the UK.
The best thing you can do is buy a testing kit. The kind you dip in the water don't test for ammonia, which you need, so a kit with bottles and test tubes would be best. That way you can keep an eye on the water yourself and know when you need to do a water change. Master test kits come with pH, ammonia, nitrite and nitrate testers - be sure to follow the nitrate one to the letter!
If you get it tested at a shop again, ask them for the numbers. Shops are notorious for saying' fine' or a bit high' when the numbers tell a different story.
When you got the tank, was the gravel left in the tank when it was moved to your house? If it was, I suspect that the previous owner did not clean it properly, if at all
Next time you do a water change, push the siphon tube right down into the gravel and wriggle it a bit. Not so much that it kicks the muck up into the water, just enough so the mess goes up the tube. Once there is no more mess, gently lift it out and push it into the gravel right next to where you just cleaned. Repeat the process until you've taken enough water out. Next time start where you finished.
Because of the state of the water, you need to be doing water changes more or less every day so you'll soon get the whole of the gravel done. Then start again at the beginning. This time there won't be nearly so much mess, so you'll get more of the gravel done each time. It won't take long to get rid of nearly all the mess in the gravel, then it will be just a case of removing what has gone in there between water changes - fish poo, uneaten food etc.
A thought just occurred tome - you said you use a sucker machine. Do you mean a siphon tube which puts the dirty water into a bucket, or a power/battery driven thing that puts the water back into the tank?