My winter bike is a Trek 1500 with a shimano ultegra 9v setup, with a triple (52-42-30 with 12-25 cassette).
Tipical winter rides are over 100 kms with a lot of uphill (max 12-15%.).
I want to semplify my crankset: a single chainring.
Here’s what I am doing:
Removing the inner and the outer chainrings from the triple cranckset, using only the 42th chainring with some washers to simulate the thickness of the 52th chainring. (Not a nice look).
Changing the rear cassete with a shimano LX (11-32)
Changing the rear derailleur with a mountan bike one (or will the long cage ultegra RD support a 11-32 cassette?)
Substituting the left brifter with a brake lever
Removing the front derailleur with his cable.
I am warried about chainline, will the middle chainring line work with all nine cogs?
Somebody suggested to change the crankset with a track-model, 42th o 44th chainring with its specific bb.
But will it support a 9 speed chain?

My winter bike is a Trek 1500 with a shimano ultegra 9v setup, with a triple (52-42-30 with 12-25 cassette).
Tipical winter rides are over 100 kms with a lot of uphill (max 12-15%.).
I want to semplify my crankset: a single chainring.
Here’s what I am doing:
Removing the inner and the outer chainrings from the triple cranckset, using only the 42th chainring with some washers to simulate the thickness of the 52th chainring. (Not a nice look).
Changing the rear cassete with a shimano LX (11-32)
Changing the rear derailleur with a mountan bike one (or will the long cage ultegra RD support a 11-32 cassette?)
Substituting the left brifter with a brake lever
Removing the front derailleur with his cable.
I am warried about chainline, will the middle chainring line work with all nine cogs?
Somebody suggested to change the crankset with a track-model, 42th o 44th chainring with its specific bb.
But will it support a 9 speed chain?

Try before you buy on the derailleur switch, you probably won't need a longer cage, since there's no front shifting slack to be taken up.

Middle chainring will probably work in all cogs, but you could have some issues with deflection on the biggest cogs. That shimano 118.5 triple BB puts the crank waayyyy out there, might cruise ebay for something shorter, but again, try before you buy.

Get BMX/single ring chainring bolts if the bolts with washers solution offends the senses, pretty cheap at any LBS

Don't change your crank, no need. Going to a 3/32" non-ramped chainring might prevent some chain drops, and would work with a 9-speed chain, but i'd stick with what you've got until continued use demonstrates a need to swap.