HELP, totally lost if I should buy new or second hand

If you think about it, one of the main reasons that you lose 30% of a bike's value immediately is - in most cases - any subsequent owner loses the guarantee (though I think Specialized and Trek have some transferability these days). It's also like buying a second hand car insofar as, basic protections against theft etc notwithstanding, used bikes are sold as seen. So all in all, there's an element of risk.

Having said all of that, though, those risks can be significantly reduced by buying through Paypal transfer. Also, for anything expensive, I'd only buy what I could inspect in person, and ideally test ride too.

Do your due diligence. Look at eBay feedback, if that's where you're buying. If the buyer claims to have owned from new, ask for proof of purchase (though be realistic; no one's likely to have kept a receipt for more than a couple of years). Ask why they're selling - does the reason seem legit to you? Ask some questions about the equipment/spec on the bike; while being ignorant of the details doesn't necessarily mean something is wrong, it could be a red flag. 

Get a receipt (the seller should want one, too). 

Above all, exercise common sense and trust your gut. If it seems off, or too good to be true, walk away. Likewise - and sorry if this is ridiculously obvious - don't carry large amounts of cash, and if you're going to collect, ideally do so in the daylight and have someone else with you, even if they stay in the car.

Finally, while it's right to be cautious, do bear in mind I know plenty of guys who enjoy trying different bikes, and buy and sell new ones several times a year. For myself, in twenty years of buying far too many bikes/frames/wheelsets second hand, I can only think of 3 occasions when there has been a real problem. In 2 of those cases the seller agreed to a refund without (too) much hassle.

All of the above but I would also say that Cycle to Work schemes and the Green Commute Initiative mean that for higher rate taxpayers new bikes are effectively half price, which I'm sure has has pushed up new prices but also means for those who can use those schemes, there is better value buying new. 

Are you still looking, Adrian? Idly browsing bikes on eBay came across this beauty and remembered your post - at £900 with no bids and a day to go very much worth keeping an eye on, if you got that for your budget it would be a very good bargain.

https://www.ebay.co.uk/itm/CERVELO-S2-CARBON-AERO-ROAD-BIKE-61CM-SHIMANO...

Generally you'll lose 30% + of a new bike's value by riding it away - the same way as you would a car.  This is enough to get you a new, improved groupset or better wheels if required - or simply a better bike on eBay.   

I'm 6ft 4, but ride 57-58cm frames with a longer stem, but there are some good 61cm frames on eBay including a Synapse within your budget.  There's also a 58cm Synapse too. 

If you're buying a carbon frame I'd limit it to collecting in person so you can have a good look at it - unless you're buying from one of the specialist eBay outlets that deal specifically in second-hand bikes....but you'll pay a bit more. 

It's a matter of risk and trust. New products have fitness for purpose standards plus retail standards providing low risk at high cost. Second hand from the trade means no manufacturers warranty but some duty of care and retail standards to offset a lower cost. Few workshops have access to non-restrictive testing of structural components, but the best do. Private sales on eBay have some terms and conditions but no requirement for meaningful quality control. As stated, many people have harmless reasons to sell, so it comes down to what risk you want to take and what knowledge or access to knowledge you have. I only sell what is as new and has been checked by competent engineers so that I'm willing to ride it myself. Not virtue signalling, just ensures I have no doubt or concerns, so selfish really.
In summary, I'd advise taking your most knowledgeable friend to inspect the potential purchase, whilst you talk to the seller and decide how much you can trust them.
Stay Safe.

If you can afford to bide your time, secondhand always for me. Road bike is a Btwin Facet 7, full carbon with complete Ultegra Di2 groupset, 2014 bike as ridden by the AG2R Mondiale development squad and the France U-19s that year. RRP £2,999 in 2014 got it for £850 in 2017 from a mountain biker who'd wanted to give road a go but didn't like it. About 600 miles on it, it was pristine and still rides like a dream for the money I could have spent on a new aluminium with own-brand wheels and Claris. Her indoors has a four-year-old Planet X Procarbon RRP £1250 that we bought for £500 from a guy who bought it to do a charity LEJoG and never touched it again. Fabulous bargains out there and often lower prices for people your size.

If you do want to go new you could do a lot worse than Decathlon's Van Rysel Ultra CF 105, £1499 for full carbon, 105 groupset and Mavic Aksium wheels - same frameset as that currently used by AG2R's under 19 squad.

New. You have to wonder why someone is selling a perfectly good bike if you are looking at the second hand market, the chances are that you will have to change something on it which is going to add to the cost, buy the best new bike you can afford 

Whatever you can find in stock! Supply chains and the second hand market are crazy.