I agree with the previous response in that you cannot really answer your question easily. There are so many factors to consider as you need to size each of your rooms first for heat loss, then decide where you want each radiator, determine the position of the new boiler before finally working out the best route for the pipework and the size to use in each section of the house.
In principle there is nothing wrong with what you are proposing as most bungalows have the same issue. I had a similar problem in my house where I was installing a new sealed CH system. The whole ground floor was concrete so the only way to run to each radiator (without having pipes running across the surface like the old one pipe system) was to drop it from the upper floor. I ran the main pipe in 22mm from the boiler and ran this across the length of the house. Upper radiators were tee'd off the main 22mm pipe using 15mm pipe, and the downstairs were tee'd off from either the 15mm or 22mm pipe in 10mm so they would fit nicely behind dry-lining plasterboard.
However, if you want to dig further then here are just a few things to take a note of irrespective of whether you take on the job yourself or get someone in:
1. Ensure the pipework is properly sized to ensure it can carry the required load to all the radiators.
2. Remember to use a radiator valve with a drain-cock built in on each radiator. This ensures you can drain the system if you need to remove/replace/repair a radiator or valve at a later date. This ensures any trapped water in the pipe between the ceiling and the radiator can be drained at the lowest point.
3. Ensure that you run the last meter of pipework or so to the boiler in copper.
4. Draw a plan of wher you going to run pipework. This will help work out how many and what type of connectors you will need, and what lengths of pipe you will need. It will also help you work out the shortest and most convenient route to the radiators.
5. Use the coils of piping rather than the shorter straight lengths. This allows you to use less connections and is the bonus with plastic plumbing. I have a 4 bed house house with 15 radiators and I never used a single elbow or a single coupler. The only conections are equal or reducing tee's.
6. Always, always fit the pipe inserts as recommended. Always ensure that connections, e.g. tee's, manifolds etc, can be got at. Yours should theortically all be in the loft which is ideal.
7. After installing the pipework, and before connecting an radiators, put stops ends on any open pipework and then wet pressure test the pipework to the manufacturers recommendation to ensure you don't have any leaks or faulty connections.
Personally I would get an "expert" in and get a quote for the whole job and then ask him whether he would have an issue with you installing the pipework and radiators, with him connecting the boiler and commissioning the system. Some plumbers won't do it, some will. Mine agreed and so I did my homework and I showed him a plan of where I was running the pipework and in what sizes before I started the work. He OK'd it and after installing it he fixed in the new boiler for £280 and I saved myself a few grand on the cost of installation.
Connecting the actual pipework together is not difficult to do and certaily a lot quicker and cheaper then doing it in copper. It's the planning and preperation that takes the time and finding a plumber who will agree to fix the boiler to new pipework he didn't fit.
Best of luck.
Big bungalow that one, 18 rads? Too many questions need to be answered here from you, get several quotes first off your local and then you may have an idea of what is involved.... 
