If I want to invest 10,000€ in specific bond with a par value of 1,000€. When I make order, what do I get (10 bonds or one bond that is worth 10,000€)?
You "get" 10 bonds - meaning that you can sell part of this lot (in whole bond increments) if you want to.
If I buy a 30 year bond that has 5% coupon yield, below par value of 1,000€ (800€) (can I make an order for 10,000€ or it needs to be a multiple of 800, that relates to first question),
You would pay 800 for each bond that you buy; they have to be bought in whole bond increments. For 10,000 you could buy 12 bonds for 9,600 and have 400 left over.
then I would get a yield of maturity (29 years) of 6.33%. For how much can I sell my bond or bonds? (relates to first question) after 5 years if the price would be 1,400€?
If the "price" is 1,400 then you can sell your bonds for 1,400 each.
Note that it's very unusual for bonds to increase that much in price. For a bond to go from selling for a discount to selling for that much above par would mean a dramatic collapse in interest rates, since your bond would be much more valuable because it pays a higher interest rate than other bonds that are issued after rates drop.