profoundly messed up aquarium

I'm cleaning up a 120 g. aquarium with some severe problems:
the tank belongs to an older man who has health problems and
severely neglected it for what I would guess is close to a year.

The setup is: planted 120 with Eheim 2228 - the filter was pretty
crudded up, having not been cleaned in at least 6 months. The
substrate (4 inches of laterite) has not been cleaned in a very long,
long time: when I used python to do water change and started poking
into it to get detritus, it released a stream of bubbles, which I
have no idea what they were (no odor).

Algae is all over: the glass is covered with something that I thought
might be cyanobacterial, but it did not clear up with doses of maracyn
and water changes. It is green-to-brown and comes off very easily with
razor; it also seems to have some gaseous bubbles trapped or generated
in it - I've never seen anything like this before.

The plants (java fern, vals, some hygro) are covered with a different
algae that feels fuzzy to the touch - it is very short and impossible
to remove. Other of the plants seem covered with something that looks
a lot like brush algae,

Somehow, the fish seem healthy - at least the tank is very lightly
stocked.

I have no idea where to begin on this. I thought to do a complete
breakdown - pitching the substrate and plants, replacing hoses and
filter intake and return, a bleach dip for the thermostat (also covered
with that fuzzy algae). But before undertaking something that drastic,
I wonder what alternatives there might be - any suggestions will be
greatly appreciated.

Thanks in advance -

Mitch Edelman