I would agree that when starting afresh, condensing ones are the best to get nowadays because although they cost more they are significantly more efficient than non-condensing. The makes already mentioned are worth looking at. The boiler output needs to match your house volume and heat-loss characteristics. Only the best built and most medern houses retain heat well. The radiators need to match the requirements room by room and should be sited under or very near windows for best results - do not be conned into accepting an installation where all the radiators are just inside doorways to each and every room: this is cheap to install but singularly poor design. The number of radiators tells you little or nothing about the necessary boiler capacity - it all depends on the total requirement for heat, radiators come in a very wide variety of sizes/output. Your local library is likely to have DIY books on basic central heating design.