Cheaper to buy or bring with me to HCMC?

How about a larger Android tablet?
Would the larger screen make things easier?

How about using a laptop connected to a wide screen LCD/LED monitor ... like this one from Dell (27'')?

I assume, you have difficulty with smaller screen size and touch-screen typing.

If you want to buy another iMac in HCM, you can compare the price range here.

Thanks for the info - I will have a look when I arrive and see which one is best for me.

I have difficulty with smaller screen size and touch screen typing

I too brought my iMac to Vietnam and regretted it.  I had the original case and brought it as excess baggage so it cost only $150 but that was not the problem.  It needed repairs twice and it took an expensive taxi trip to bring it to downtown.  I also think it ran rather hot in Vietnam, possibly because the screen was running on 220v.  The newer iMacs reportedly run cooler. 

My advice would be a Mac Mini or, as senwl suggested, a MacbookPro attached to a monitor.  I know that all Apple products are a lot cheaper in the US than in VN but I don't know about the UK compared to VN.   You can go to the Apple website and switch to the VN store to check prices. 

If you buy a Mini in the UK, use your TV screen at home as a temporary monitor.  Once you transfer everything from your iMac to the Mini and bring it to Vietnam, you can buy a monitor after you arrive.   Also be sure to use an Uninterruptible Power Supply (UPS) to modulate the power.  You don't need to do this with a laptop as the charger is an AC>DC power converter but you should use one with either an iMac or a MacMini.  You can buy one in HCMC as it would be too heavy to bring one conveniently.  Poor power quality turned out to be the source of my problem.  It always worked fine in D1 when I brought it in but gave trouble at home.  The UPS solved the problem.

If you decide to buy an Apple in Vietnam, you can simply back up to a portable HD and restore to your new computer.  There are different software ways to do this but different people have different preferences.  I like Carbon Copy Cloner. I carried multiple backups in different pieces of luggage to be sure, even though I was moving the computer at the same time.

I did find a very good Apple authorized repair guy in HCMC.  If interested send me a PM for his contact info.

Thanks for the info - much appreciated

I don't know if it's cheaper here so I cannot help you with the comparison.   As how to bring desktop computer overseas the least expensive way, I had done it twice (to and from) as checked luggage.     

What I did (actually, my spouse did, I only watched) was packing it in the original box (if you don't have the box anymore, find a substitute at a UPS store), then putting the box in a larger box, padding it tightly with foam over, under, and around the smaller box.  Finally, he packed the whole thing in a hard shell luggage with more foam and bubble padding.  The computer was tightly cocooned in so many layers of foam by that time, it may have survived a tornado even.  At the airport, we wrapped the suitcase in plastic wrap, paid the insurance, and dropped it off as part of our free checked luggage.

The only cost involved in the shipping was foam, bubble, and plastic wrapping.  The best thing, however, was no customs duty and no delay while producing documents to prove the electronic item was used and ours.

Then again, all the posts above show ways to suit your need even better.  My comment is only to share my experience on the packing and shipping.

Sorry but I gave you some dated info.  What else is new. 

Apple used to list VN prices but not let you buy online.  Now they no longer list prices and products but direct you to their authorized reseller list.  Even then you need to check carefully as most of them only sell iPhones and iPads.  Perhaps Tuyen could find out the prices in VN if you are careful about giving her the specs you want in writing and have her bring them to one of the resellers that handles Macs.

I was lucky that I kept my original iMac box but I would add one caveat to what Ciambella says about packing your own.  Be careful about dimensions as well as weight.  The original box was just barely at the maximum for international luggage (perhaps planned that way by Apple.)  If you go over dimension maximums, there are charges added to the basic excess baggage charge.

Thanks again for for advice - I am visiting Saigon/HCMC later this month for a two weeks, I'll have a look and see what is best option for me

Yeah I would never transport an iMac, too big, and you don't want the gorillas throwing it around. You will have enough other junk to pack. Sell it. Vietnam prices for Macbooks are US retail + about 10%. I assume iMacs the same ratio. UK prices probably in the same ballpark.
You must already be backing up with Time Machine to an external disk,   carry that with you. I also back up the cloud and recommend Backblaze.