Royal Enfield Classic 350

Considering your use case, we would advise choosing between the TVS Apache RTX 300 and the Hero Xpulse 210. The RTX is an adventure bike, which will make it great on the road, while the Xpulse is a dualsport, which will be more capable during serious off-roading. 

If you want a more road-biased machine, then the Honda CB350 H'ness (or either of its siblings), the Royal Enfield Classic / Bullet 350 and the TVS Ronin would make for a good option. 

Take a test ride of these machines to help decide which one is best for you. 

We'd suggest that you look at a newer bike, primarily due to ethanol-blended fuels that are likely to cause problems with older motorcycles, even if they have been refurbished. We suggest you look within the Royal Enfield family itself at options like the Meteor 350, Classic 350, or Bullet 350 to meet the requirements that you have highlighted

For your usage, in the cruiser space, you can take a look at the Royal Enfield Classic / Bullet 350 as well as any of the three Honda CB350 models. All bikes have relaxed, smooth engines, spacious seats and great suspension comfort. 

In the ADV space, options like the TVS Apache RTX 300 and KTM 250 Adventure can be considered for their punchy - but not overwhelming - performance, spacious seats and comfortable ride quality, as well as luggage carrying capability. 

Take thorough test rides of all shortlisted options before you decide.

The TVS Apache RTX 300 and KTM 250 Adventure will both fit the bill for your needs. Both have spacious seats, will return decent fuel economy, have great suspension and ride comfort and should be light on the pocket. Do bear in mind that these bikes are significantly more powerful than the Unicorn you currently have, and if you want something that's more laidback, there are other options. 

Bikes like the Royal Enfield Bullet / Classic 350 and the three Honda CB350 models will also fit the bill if you don't mind their laidback character and retro looks. 

We recommend a thorough test ride before you choose. 

Since almost all of your usage is going to be in the city, going for the Royal Enfield Classic 650 doesn't make sense because its rear suspension is quite stiff, and in traffic, its 243kg kerb weight will be a task to deal with, especially considering the way the engine will splay your legs out. You are right to worry about fuel efficiency because the 648cc twin is not very fuel efficient at slow city speeds, especially considering the bike's 243kg kerb weight. 

The Classic 350 is a lovely bike to use inside the city with its upright ergonomics, plush suspension and torquey, soulful engine. The Goan Classic is also something you can take a look at if you like its custom bike look, although it does trade off a little bit of practicality and comfort to the Classic.