Combi or not ?

I am self building a new bungalow, all the pipework is in for a combi in the roof space.
I am now getting nervous re combi, question is, is it still practical to change my mind and put in system boiler and a cylinder, I do not think funds will run to a pressurised system, other query is I have already fitted a Mira Excell thermostatic shower to run of a combi, could I still use this if I go with a system boiler??
Finally if it is practical to change ball park cost anyone to help me decide.

Expensive because the only option is unvented - consider a thermal store perhaps.
Shower is designed for high pressures..

There are lots of happy combi owners; just get a make with respect, and a high hw output one. Take the mfrs extended warranty for a few years - until they've got the bugs out of the design

Whats putting you off a combi?? I have had one in my house for 19 years (im on my second), It has happily kept my wife and three kids warm and washed for this time.If its water flow your after just got for a 4okw worcester of something of the like...mine will fill a washing up bowl and run a shower at the same time no probs.

Why change indeed! Get a good combi (Vaillant / Bosch etc) I deal with Vaillants over 20 years of age and you can't wear them out - just the rubbers go after around 14 years.!