Every good heating system starts with a heat load analysis- size matters, both for efficiency, but also for reliability. An oversized beast- even a mod-con will see an order of magnitude more firing cycles than one more appropriately sized for the load. If the system is cut up into multiple zones it's even worse. Every ignition cycle & flue purge is both an efficiency hit and adds some wear to the system components.
If you have all the numbers off the rating plate of the older system (or make & model #) and a year's worth of fuel consumption data and your ZIP code (for weather data and heating design temperature purposes) it's easy to calculate an upper bound on sizing. The Ultra 230 has gia-NORMOUS output compared to the actual design day needs of average sized homes, or even reasonably-insulated mansions. (It's about 7-8x the boiler I'd need for my ~2200' +1500' of semi-conditioned basement 1920s antique bungalow in central MA.) Unless you live in a 10,000' leaky 2x4 stick-built with only single-pane double hungs (no storms) and no insulation, or you're heating your swimming pool with it as well as the house, I'm betting that you can downsize considerably from the 230.
Then there's the issue of radiation type, number of zones, size of the smallest zone, etc.
The size & type of the indirect tank will depend on the boiler size and the number of people or anticipated peak loads.
There are many good mod-cons out there and only a few real dogs- local distributor support & expertise are more important factors than who built the boiler. A really great German or Japanese boiler isn't worth squat if spare parts and expertise are thousands of miles away. (In the mid-Atlantic states, NJ-based Triangle Tube has a huge fan-base for there Prestige Solo line.) To figure out who should be designing/installing the system it's often a good idea to get a few contractor references from the distributor, since they will know who installs a gazillion of 'em and never needs to call the technical support, and they know who keeps calling them back with screwed up systems & components. Being just down the road helps (as it does in Jim's case.)