It looks easy enough to knock up a puller/ press using a long bolt and some washers.
Combined with bearings not from a bike shop, should cost about a tenner a wheel.
Is this a bad idea?

Nah, unless you’re a monkey. Careful with pullers, you really want to only apply force on the outer race, which a washer/bolt setup won’t quite do. Also rear hubs will normally need the axle in which rules out that approach.Sockets is what I use, there’s obviously other alternatives.

Worth searching for a “how to” for your particular hubs before hand, but no reason not to, go for it.

Definitely worth doing yourself. I just drifted mine out with a piece of bamboo (so you can’t put too much force through it at any one time) and a hammer. Rubber mallet to carefully fit the new ones. I’d use a socket but don’t have one the right size.

I warm the hub with a hot air gun before I push the bearings in as well. Just opens up the hub a bit and lets the bearings push in a bit easier. Don’t heat too much though.

In the past I’ve drifted the old ones out with hammer and screwdriver, bashed in the new ones using a hammer and stub of broom handle whittled down to size.

I did my Superstar hib bearing DIY. Knocked out old bearings with screwdriver and hammer.

Used threaded rod, nut, washers and old bearing to push the new bearing in. worked fine no problems.

Cool, cheers