I'm in US - and the equipment and practices are a bit different here, so keep that in mind.

A standard atmospheric vent gas water heater is about 65% efficient - max. In addition, it basically has a 3" hole from the inside of your house that is open to the outside and a thermal air pump that moves your heated air out of the house 24/7/365.

I don't know what a "standard" boiler there looks like, but if it doesn't use a combustion blower and sealed (usually stainless steel) vent pipe, it's not 85% efficient. An atmospheric vent boiler is similarly going to be about 65% efficient. Above 65% any gas appliance needs a combustion blower to vent the flue gasses. Above about 82% the flue gases are cool enough that the water vapor in it starts condensing (that vapor is acidic and eats metal vent pipe, that's why it has to be stainless steel). At 90%+ efficiency a gas appliance is vented with plastic pipe.

Similar to the atmospheric vented water heater, an atmospheric vented boiler (and most 80% boilers) will pull combustion air from your house and move it outside. All of that air has to be replaced with cold air.

I'm a big fan of 90%+ efficient gas appliances - not only for the lower operating costs, but because they often solve venting problems also. Plastic pipe venting is a wonderful thing. They also have outdoor air intake so you aren't using inside air for combustion air.

I would look at a condensing (90%+) combo unit or condensing boiler and standard tank water heater. Your boiler is the most expensive thing to operate and that's where you want to focus your money if you have limited funds.

My experience is that the condensing boilers were a little iffy 20 years ago - with much of that being install issues - but are very reliable now.

I recommend high efficiency with high quality stainless heat exchanger and tank for the boiler. I also recommend an indirect hot water heater made of a high quality stainless steel inter tank and heat exchanger. Quality is the determining factor in longevity and the right purchases will last your life time and maybe your kid's lifetime.

Small components on the boiler may need replace like inducer fan motor, pumps, some controls or valves but the many body will always stand the test of time. An indirect water heater is just a storage tank made of stainless and the water is heated from the boiler by a stainless steel heat exchanger in the tank. No direct fired heat is used heating the water in the tank, no burners, controls, flue exhausts, nothing. Just a storage tank and once heated standby loss is very small.

The indirect water heater costs more than twice a standard water heater but it will last longer than a dozen standard heaters. The worst thing for a boiler is sitting idle for months during the summer, with this set up the boiler will continue to fire up to supply hot water over the summer months reducing initial startup problems when you finally need heat. All your hot water will be produced at the same efficiency as the boiler rating, so get the 95% boiler.