Which strimmer?

Go for the best one you can afford and that is the right weight for you. We have two, his and hers, his is an Echo and too long and heavy for me so I have a shorter Sthil. Still quite a weight but if you get the harness properly adjusted then it makes life much easier. Stick to a known make as you can easily get them repaired and look at several before you buy.

I presume you want to use it more as part of your job than at home/allotment ? You say you have only used one once before, why not hire one for the odd occasion it's needed ? save on initial outlay & they get to do the maintenance.
As to cc's that depends how long & how often it's used & what it's used for, the choices you show both have strimming & brush cutting heads, brush cutters need more power to work efficiently than strimmers.
I have a split shaft 28cc Ry0bi which is fine, but it's only used for about an hour a year on my borders & it has only got a strimmer head.

I do a few job for other people, but I do see this as regular work, when I am not too busy, I have a proper job too

Its reading reviews that put me off, and I like to buy the best that I can. I have a multi-tool, but that is waiting to be fixed.
I will have lots of free time in the school holidays, I hope to earn enough to pay for my tools

I am picking up a petrol hedge trimmer today, Tesco direct, £135

The strimmer that I have is 25cc, the one I may be buying is 52cc, I am wondering if that is overkill. Motorbikes are 50cc, do they really need all that power. Although some Stihl strimmers, top brand, are larger than 25cc.

The cheap ones seem fine....for normal use

I have had a few cheapo strimers, all have worked fine unlike people said,
1st one i used for a year from b&q i think it was 28cc `mac power tools brand, used every few weeks doing elder and brambles and eventually the handles broke with me hitting giant lumps of clematis growing over trees up in the air with the brushcutter blade on... as they had plastic connectors,
The second one (B&Q) own brand with orange plastic bits 25cc engine.... no idea how good it was it seemed ok but it was stolen but did have metal handle connectors,
Third one ive had about 2 years raider tools / Baukraft ( from Bularian diy store Baumax/Homemax ) (looks identical to china type b&q orange plastic one ), 22.5cc engine regularly used (almost every day i cut between 500 and 1000sq meters) for normal striming of nettles grass, elder, giant hemlock plus brush cutter blade for pruning trees... and other things,.and brushcutting the soil to remove weeds and the roots....2 years of it and it still starts and works fine

Only problem with any of them was the first one with plastic bits holding cross bar of the handles on and the plastic holding the blade guard on ( all brands now seem to be metal )

Another problem with both the first one and the third one was after a lot of use they refused to start, in both cases the carb / intake had come loose and 5 minutes tightening it up and they were fine, came loose again added some of my threadlock adhesive and the carb had stayed on afterwards so they start fine every time.

If you're buying a hedge trimmer and looking for a strimmer, why not get one good tool to do both? Interchangable heads/tooling on one good reliable motor is less storage/carriage and you only have one engine to service in the cold season.
+1 on the idea to buy 'branded' tools like this too, getting parts for a chinese cheapie next year will be more trouble than the dealers want to know. Generic things like starter string and primer bulbs are OK but anything else and it can get hard going.

I would (and do) buy tools like this on ebay. Top end tools which even if they've had a hard commercial life have mostly been better serviced than i will ever manage... Plus, as mentioned above, if something goes wrong, you can pick up spares easily.

I don't really understand buying things like this new... They're only going to lose value and get dirty and grubby!

As the work is regular you might as well buy the best available, it will of course be tax deductable against your business expenses

I bought a Stihl FS38 which was a Which Best Buy a year or so back - it is very light weight so suitable for all for trimming grass edges on the allotment - we also have a heavier Stihl model to deal with overgrown areas which is used mainly with the brushcutter head but after 40 mins you can feel it even with the harness in use. :-)

Only if you earn enough and are registered for VAT unlikely if its a school holiday job

Just as unlikely if it's a 'cash in hand job', but equipment depreciation can be claimed on low earnings

I have now won a strimmer on ebay, £80, listed as new and the same as this one from SGS

52cc 5 in 1 Cutting Multi-Tool Set | Gardening | SGS Engineering

The SGS strimmer seems to have increased in price, this week, by around £60; maybe its because the pound is at a record low value.

I have received a message from ebay that the item I bought has been withdrawn, due to account being hacked, they have said that I should get my money back, even though it has been marked as dispatched.
Bargains are often not what they seem

That sucks!