Some things you should consider when buying a Macbook. iMacs remain decent value, but the new Macbooks are rather overpriced.
1st: its a big purchase, so research for yourself - Google 'macbook buying guide' for lots of more detailed guides.
BUY SECOND HAND OR REFURBED
The latest Macbooks are overpriced, and the trackbar is a rather foolish addition. IMO.
Apple lists several refurbed models, which are one option, but eBay and other sites for 2nd hand Macs are a good choice. You could simply ask in local Apple retailers too - try repair shops.
Obviously you should consider issues with guarantee (or lack of) when buying 2nd hand. If you can add AppleCare (extended warranty) its arguably worth it: expensive, but peace of mind.
I'm going to focus on Macbooks, but most of my points below apply equally to desktop iMacs.
MACBOOK PRO, AIR?
The Air is a consumer model not meant for processor-straining video editing work. Only look at Pros.
The 12" is a toy. I've edited a 90min film on a 13" MBP and another on a 15" MBP. Its much easier with the larger screen, and you can avoid having to scroll back and forth so much. But you pay a price premium for the larger screen. If you'll often use it at home you could consider outputting to a 2nd monitor.
HOW OLD A MODEL SHOULD I CONSIDER?
I still use a late 2011 13" MBP, which works fine - since I added RAM. Thats a reasonable oldest model to consider.
If you're not sure about a model listed somewhere, check it against Apple's model list (or the Wiki, which gives a clear breakdown by year, eg the 2012-2015 list).
Eventually its the video card that won't be able to cope with new software - I've had school iMacs that couldn't operate Final Cut because the graphic cards (circa 2008/9) were too low powered.
This is the factor that will most influence how fast your Mac works. 2GB and you'll be seeing a lot of the spinning wheel. 4GB is the utter minimum to consider.
You can buy compatible RAM upgrades fairly cheaply, and follow a YouTube guide to install yourself (or pay a shop to do so). I went from 4GB to 16GB and the difference in the speed of my 5 year-old Mac is extreme.
Most have an Intel i3 (slower), i5 or i7 (faster), with a variation of duo core (faster than single, but slower than...) or quadcore (faster). Octacore are appearing. Thats 2, 4 or 8 separate processors within the CPU to speed things up. The price rises accordingly. Prioritise RAM, but a quicker CPU will help speed up video rendering etc.
HARD DRIVE + PORTABLE DRIVE
This can also be upgraded - there are many online guides and tutorials, or you can pay a shop to do so. Ideally, you'd want a 1TB drive, as video files quickly add up.
You'll mostly see 500GB drives though.
256GB is worth considering ... IF its an SSD. That means a Flash drive, which is much faster than a traditional HDD. Video editors should have a portable (powered by USB 3, NOT a plug-in model) of 1TB minimum (ideally 4TB). I have blogged on formatting these.
I did find when editing films that even a 500GB drive kept running out of space - but that was a 90min documentary with many hours of clips. I had to export some edited sequences, and use those mp4 files, deleting the source clips, to free up space, as the render files were huge. (Final Cut Library Manager can help with dealing with that).
From around 2013 Apple stopped including a 'Superdrive' in MBPs. You will need to buy a USB superdrive (typically expensive!) or ... buy a cheaper generic alternative (some examples).
If a MBP comes loaded with software, its likely to be pirated copies that as well as being illegal you will never be able to update. Final Cut Pro X is expensive (about 300e), but you get all future upgrades free. You'll see a button for free trial here as well as to buy. There are other programmes you could get at the same time - just read Apple's guide.
There are risks with 2nd hand, but thats where the value for money is - the new models are very overpriced.
In time the video card will not be able to cope with new software, but you could at the time of writing go back as far as late 2011 models.
Screensize is a factor: its easier to work with 15" but its also heavier and more expensive.
RAM will have the most impact on the speed of your Mac. 4GB minimum; 8GB or 16GB is much better.
CPU is important; ideally you'd want an i5 or i7.
A Flash (SSD) hard drive is faster, but as they're so expensive it will be smaller. If its a traditional HDD, 1TB will make life easier, and 500GB should be a minimum.
You can upgrade RAM and HDD. You should buy a portable HDD too.