It's handy to have the boiler fitted before you install a new kitchen. You can pull down and take to the tip (or use in the shed) any old cabinets that are in the way. There will be cables to run, as well as the pipes. Once boiler is fitted, you can assemble the cabinets round it (there will be clearance required for maintenance) and if you assemble them with a standard sized gap (e.g. 500mm wide) you can conceal the boiler behind a matching kitchen door. It doesn't need a full cabinet with a top and bottom, just a door.
Boilers are usually deeper than standard wall cabinets, but you get get extra-deep wall cabs for this purpose. My boiler is in a corner, and due to the cooker hood, has only one cabinet next to it, which I had made to fit. This looks better than having just one deep cabinet sticking out from a row of them.
BTW, put your boiler where it will be close to the waste pipes for the kitchen sink, so the condensate pipe can be plumbed internally. Many people end up with an extrernal pipe, quiicker and easier for the installer, but freezes in cold weather and your boiler stops workingt. This is very tiresome.
A modern combi will throw out a great cloud of steam in frosty weather, so try to place it where this will not blow across your window, or towards your neighbours house. Close to an external corner is handy as wind swirls around and disperses it. You can have an extended flue (like a chimney) or a plume diverter to carry the plume past windows and doors. Easier and cheaper if specified prior to boiler installation.
As for cabinets, they are usually hung with adjustable brackets onto steel plates screwed to the wall. You can get cabinet hanging rail which is installed in a long strip running all along the wall, and you then hang cabs whereever you want them. This is very handy as you can reposition them with ease and no more screws or drilling required. A bit like, if you have picture rail, you can move your pictures an inch or a yard with almost no effort. And they will all be level. It is very strong.