"The hot water takes at least two hours to be hot enough for one bath "
with respect, you are making the mistake of assuming that because you have an inefficient old boiler, you need a combi.
What you actually need is an efficient and modern CH and HW system.
If you only have one bathroom, and mostly have showers not baths, a combi may be sufficient for your needs. But it is likely to be slow at filling a bath. Will anybody ever turn on a tap or flush a WC while you are having a shower?
A modern boiler can heat a modern cylinder in about 20 minutes, and people now often choose cylinders big enough to run two baths.
What is the colour and size of your old cylinder?
Before deciding on a combi, please test the flow and pressure of your incoming water supply. Fill a bucket at your kitchen cold tap (and your utility room or garden tap, if you have them). Time it. How many litres per minute does it deliver?
Now do the same at your hot bath tap.
What are the lpm numbers?