To answer the specfic questions you asked;
1. Can you convert a double to a triple (New cranks or new shifter/derailier/cranks)
– you will need to buy a new chainset, shifter and front mech
2. What is “compact”. I guess it’s just smaller chainrings, i.e. You can convert to/from compact by buying new chainrings
– your right compact is a smallet of chainrings, you cannot convert a ‘normal’ chainset to a complact and visa versa. A compact chainset with two rings will give you enough gears to get up hills without using a triple
3. Will a 500 quid Allez wheels be able to take my ‘bulk’
-Yes, depandant on how ‘big’ your are and how inclined on riding through and not round pot holes you are
4. Is there anything as cheap that is super strong (not light)
-Most of the major manufacturers will do decent bikes for that money, Ribble training/winter bikes are generally good value and sturdy (not sure how much though), theres always second hand.
5. Will this end up being a waste of cash?
-Definatlety not,