I want this project to have one power supply, but the motors run on 115Vac and the arduino and relay board run on low voltage DC power. Is something like this the best option? Or is there a better way to do it?

 

That should work.
If you have any relays, they could be supplied by the 12vdc.
Max.

 

So I picked up that power supply noted in my link above and its waaaay tooo big! There has to be a power supply out there with a small form factor no? Could I take a standard 12v AC-DC power adapter and split it open and wire for my use or is that just dangerous and asking for trouble?

 

So I picked up that power supply noted in my link above and its waaaay tooo big! There has to be a power supply out there with a small form factor no? Could I take a standard 12v AC-DC power adapter and split it open and wire for my use or is that just dangerous and asking for trouble?

I have done this a few times, make sure it is a switching supply and not a linear version as the former is generally a regulated supply now.
If you re-encase it in an enclosure then there should be no potential danger.
Max.

 

I’ve mounted wall-warts inside cases several times. I have a collection of freebies from old tossed out electronics. I mount them with either a bracket and screws or heavy duty wire ties.

Sometimes you can find notebook power supplies that have multiple voltage outputs. These are easier to make nicer connections because you can cut the power in & out wires and either crimp or solder them.

 

What would be better, aluminum or plastic enclosure? I found a wall-wart that I stripped out of its casing. The bottom has all the soldering so I wasn't sure which would be better.

 

What would be better, aluminum or plastic enclosure? I found a wall-wart that I stripped out of its casing. The bottom has all the soldering so I wasn't sure which would be better.

Not really sure what you are doing but metal is conductive and allows heat to transfer through it better.

There are thousands of solutions to use to power an arduio/relay board.. But you didn't give much info to go off of so you aren't getting good answers.

How much current do you require at 12V?
Do you need other voltages besides 12V and 115AC?
Size? aka define "small form factor" with actual numbers.
Are you going to enclose it?
Will your enclosure be ventilated?
etc..

 

Some differences that come to mind.

[FONT=Verdana][FONT=Verdana][B]Plastic                                    Aluminum[/B][/FONT]
[FONT=Verdana]Cheaper                           Higher cost[/FONT]
[FONT=Verdana]Low ESD susceptibility        Will attract ESD, so you may need to ground the box.[/FONT]
[FONT=Verdana]Easy to modify                  Will require metal working tools to modify[/FONT]
[FONT=Verdana]An insulator for heat           Can be used as a heat sink but will also dissipate internal heat.[/FONT]
[FONT=Verdana]Impact resistant                 Dents easily, [/FONT]
[FONT=Verdana]Moisture resistant               Can corrode with moisture[/FONT]
[FONT=Verdana]RF passes through              Acts as an RF shield[/FONT]
[/FONT]