Need advise on hot tub wiring/utility locating

Looking for some advice rerunning my hot tub wires on a home I just purchased. I'm going to be doing the job myself. I'm not an electrician but I sell pressure washers and have wired to disconnects for my equipment. I'm not paying someone to do a simple job. Now here's what I have. Right now it looks like the electrician that was hired to run the hot tub wires was drunk or high. They did a terrible job and used direct bury (the flat stuff) in either a 6 or an 8 gauge looks like 8 to me. They ran a 4 conductor wire and only used three wires 2 hots and a ground. No neutral (not sure if u need one) They drilled a flat hole inthe side of the house and just let in fall down the side of the exterior wall and them left the wiring exposed on top on the ground. Then they ran that to a disconnect about 40 feet away from the breaker panel. From there they ran more direct bury Under some very small gate material that the pool deck drains into for water runoff. As soon as I saw this when I bought the house I immediately shut the breaker off to the hot tub. So here's my plan and questions. First I'm going to purchase new wire or try to save the 80 bucks and cut the direct bury and reuse the wire provided. Then I'm going to use the grey pvc probably 1" I think it's called conex or something like that and run it to the disconnect underground. Question how should I locate possible underground plumbing or electrical from the pool? Also how deep should I make the trench? What size wire should i use for a 40' run? I believe the breaker is 50amp. Then I was going to move the hot tub somewhat closer to the breaker outside (need to know how far for safety and code) so that I don't have to keep the wires going through the water drainage grate. I'm wondering if I should just hand dig the trench so I can be careful of possible plumbing and electrical. (I know there's a 2" PVC line about 4 feet away for the drain on my sand filter and there's electrical for the pump somewhere close to my dig) some of these questions I know maybe answered best by my local electric component supplier (have an account at border states electric and summit) but I figured the guys that do it everyday might be able to better assist than someone that sells parts. Please don't give me a hire a professional answer. Everytime I hire someone to do something they do half arse work and for me I'd rather do it myself and know what was done and how it was done in case I have problems down the road. Any other pointers would be greatly appreciated!

Thanks,

Rob