The most important is checking oil pressure, easy if a bike has an oil pressure gauge installed, much more difficult if not. Second measurement would be compression, it should give you some picture of engine performance and shape. Engine is always mechanically noisy, could be scary for somebody with no experience with triples. If the bike is in riding condition, I'd always try to ride to have some idea about gearbox and clutch condition. I wouldn't worry about poor brakes, no charging,electric start or excessive gas consumption - these problems have to be rectified with any project by a new owner and are easy to solve.
Is it original?---looks to be to me. Added on windshield crash bars and panniers but without those--very original I would say. Is it worth it/ I have no idea of the market for 40+ year old Triumphs in Tenerife but in running order $7K would be a good buyers price in the north east of the US. The larger tank is good--more range and more bulk to deaden the noisy engine. Red rather than yellow tank? Yes--I don't know what the proportion of red: yellow was but from my observation over the 45+ years that I have been dealing with these beasts the ratio is at least 10 to 1. Here in US the yellow tank is a very rare beast indeed. What to look for/ Assuming the bike runs then check that it idles smoothly once warmed up. Then switch off the engine with one eye on the red oil pressure warning light. If it stays on for a couple of seconds after the engine is switched off the odds are that the crankshaft bearings are OK. With the engine running turn on the headlight. Then rev the engine up and down. You should see the headlight getting brighter as you increase the revs and a bit duller as the revs decrease. If this happens then the charging system is probably OK. Look at the discs-they should show even areas of shiny material (check both sides of each disc. If so then the brakes are probably working as they should. Then the things that are not specific to a T160; Bike on center stand, get someone to weigh down the back of the seat so that the front wheel is off the ground. Swing the bars from left to right to the full extent of their travel. They should move smoothly with no notchiness. If there is roughness then the steering head bearings are probably shot. Then grasp the two fork legs and try to move them fore and aft. There should be no movement--if there is then the head bearings need adjustment at least--maybe replacement. Then move to the rear end--bike still on center stand. The front wheel should be on the ground--the rear wheel should be off the ground. If the rear wheel is not off the ground then the stand bearing surface/fasteners need attention. If not off the ground then put a piece of wood under each stand bottom. Rotate the rear wheel and see if it runs smoothly. Grasp the back of the rear wheel and see if you can move it sideways. If you cant then the rear wheel bearings and swinging arm bearings are probably OK. If there is movement then grasp the swinging arm and see if you can move it sideways. This should enable you to narrow any movement down to either wheel bearings or swinging arm bearings. Make sure that the bike starts on the electric start--if not then that could be quite costly. The above checks are not conclusive but can easily be carried out at the roadside without special tools and any genuine sel;ler should have no objections to them being carried out. If he has objections then walk away. Some of the potential faults can be costly to remedy whereas others are perhaps just a case of adjustment. But all of them help to give you a view of the condition of the bike and the standard of maintenance given it by the current owner. And even minor problems can be useful negotiating points to get an even better deal. Go armed with greenbacks and a trailer--do a deal on the spot, opay the money and take it away before the seller changes his mind. And--best of luck!--let us know how you get on!
A "1970" T150 is only a real '70 if the frame 'n' engine date codes are "DD" or "ED" (and the numbers match); they made less than 300 of them.
If the pictured T150 really is a numbers-matching genuine '70, then it depends what you want to do with it. As-is, it's a bitsa; if you just want to ride it, up to you how much you spend on it to get it how you want it.. But, if and when you want to sell it, it'll be worth only bitsa T150 money. Otoh, if you wanted to restore it to original spec., lotta money but it might be worth it.
Your picture won't enlarge so it's hard to be precise about small parts. But the large parts I can see that should be '70 ... aren't.