Need some help, gota put central heating into my house i just bought, but there's so many boilers i not sure. Just thought i go get a combi and bob's your uncle!

There's no central heating in the house at the mo so need it to heat that, heat hot water, theres no room for a hot water tank, but i want constant hot water dont want to be having a shower and someone's turn tap on down stair's and next thing you no its a cold shower, heard but mega flows not sure what they do!

Any help??