The KTM 250 Duke you are considering makes for a great one-bike garage to handle city and highway duties both. It has a punchy - yet not overwhelming - amount of grunt, engaging handling and will also return good fuel economy. 

Coming to the Triumph Speed T4, depending on how you ride, you should see decent fuel economy even inside city traffic. Since the bike is made by Bajaj, it shouldn't be too expensive to maintain. The advantage of the Speed T4 over the Duke will be its more effortless performance and more comfortable seating and suspension. 

And since you are shopping in this space, the TVS Apache RTR 310 is also worth taking a look at. The RTR 310 is a feature-rich machine with good performance and handling. Also, we will recommend you get the variant with the quickshifter as it is still priced sensibly, and this feature will enhance the riding experience. 

Since these are different machines in the riding experience they deliver, we'd recommend a test ride of all three before you decide. 

In the price range you're looking at, there are a variety of options in different body styles, so we'll list them out for you. If you want a sporty road bike, then options like the KTM 390 Duke, Royal Enfield Guerrilla 450 and Triumph Speed 400 are the way to go. Do bear in mind that the KTM can feel quite cramped for taller riders and, over longer stints in the saddle, this will become even more pronounced. 

If you are leaning toward an adventure bike, you should be looking at the TVS Apache RTX 300, Royal Enfield Himalayan 450 and KTM 390 Adventure models. The RTX will feel the slowest because of its smaller displacement, and it also weighs 180kg. 

Since you have a long commute from time to time and are a taller person, we would recommend you steer clear of sportbikes and cruisers. Also, stick to roadsters or adventure bikes, as they will be more comfortable. 

A test ride of the above mentioned options - and anything else you may shortlist - will help you decide which bike is right for you. 

Unfortunately, within your budget, you won't find a brand-new machine from a reputed Japanese or European maker on the market today. In fact, some of the most affordable big bikes in India, like the Kawasaki 650s, Triumph Trident 660 and the Honda CB750 Hornet, all start above the Rs 7 lakh (ex-showroom) mark, with GST 2.0 partly contributing to this.

If you are open to the idea of a used machine, you will find Kawasaki Z650 / Ninja 650 and perhaps even a Honda CBR650F in your budget. Do bear in mind that you need to be cautious and do a thorough background check on these machines. Because a replacement mechanical part will cost you a pretty penny. And these older bikes also won't be E20 petrol compatible, so that's another factor to consider. 

A radiator guard is a good idea for any liquid-cooled motorcycle - like your Triumph Scrambler 400X - especially if you plan to ride off-road. It will protect the radiator from stones and debris that can be kicked up by the front wheel, which can damage it and cause you an avoidable expense. 

If you get your bike pressure-washed often, a strong stream of water can also bend or damage the fins of the radiator. So, the guard will also help you in this situation. 

A well-designed radiator guard from reputed brands or the company's accessory catalogue itself will not affect the bike's cooling in any way. It is designed to allow air to flow through it to cool down the hot coolant, so the radiator can do its job efficiently.