Hi Sarg,
Welcome to the hobby!
a 3x18x18 will allow you to go a few interesting things and keep alot of the smaller fish quiet well
We use weirs instead of bulk heads because first up they are easier to keep quiet, bulk heads require 20cm behind the tank so we generally prefer to have a 20cm deeper tank and then have a 12cm deep weir taking up a small corner of our tanks :) also a weir allows you to skim off a thinner layer of surface water where proteins and wastes are concentrated.
Heat and power consumption go hand in hand, there is a whole range of low power items now that do a far better job than what you would have been using 10 years ago. There are propeller based circulation pumps such as the Tunze 6045 which use 6w of power and turn over 4500l/h before you would have been using power heads that use 100w of power to do the same thing. Jager heaters use nearly half the power of other brands to achieve the same results. LED lights are the biggest saver for most of us but they need to be quality, you can now use a 50w light that will yield better results than a 250w of previous technology.
During a 40* heat wave tanks will wipe out unless steps are taken, as mentioned some A/C their home which generally requires it to be used in conjunction with low power options as outlined above, which if your worried about power is taking a global solution to a local problem and you will be using 7kw/h as opposed to 200w/h if a chiller was used. but has the added benefit of making you cozy. You can have fans blowing across the waters surface that will drop the tank temp by a few degrees and in most cases is sufficient but evaporates off massive amounts of water and is only effective in low humidity. Ice blocks and then generally used. Incorporating chillers are really the only lazy and safe option that gives you full piece of mind (IMO) but can be used in conjunction with fans to reduce how often they are required.
Some people take their chillers offline in the winter as they are not needed, but its difficult to take them on and offline depending on the days you would have the clean the water out and flush it before use each time you stop the water circulation for any extended period of time as hydrogen sulphide builds up in there due to decay and is poisonous to most living things including ourselves. There is thermostats fitted to most that disengage the compressor so they only work when they are required and only use a few w's of power when in this standby mode, your only power usage will then be the pump you have feeding it, which if selected appropriately is SFA.
Ebay isnt a bad way to buy, there is also a range of
grey importers that sell them very cheaply compared to LFS.
My best advice is to take things slow and buy decent gear and do things right the first time as it will make everything that much more enjoyable.