Ahuja - avoid, they are low end and probably intended for public address use. Back in the late 70s, early 80s, Delhi used to be famous for radio spares and I remember these had the Delhi based Chawla Mechanisms which were absolute junk.
Custom made - not worth the trouble. Good custom decks were usually fabricated using mechanicals salvaged from original cassette decks manufactured for the mass market by the usual suspects. Avoid these, the assumption is usually that if something goes wrong, its easy to fix. The reality approach is usually, if something goes wrong, just throw it away because its home-made anyways
Yes, if you own vintage electronics, its a huge challenge in places like India, to find people who can repair/service them. Even in mature vintage electronics markets like the United States, the challenge is with the cost of repairs. It is extremely expensive to service or repair vintage equipment out there and service technicians usually charge by the hour.
Back to your point, like you, a lot of us set out to buy a cassette deck at some point in the very recent past. You can still get a good deck out there but here are some points I kept in mind while searching:
1) No quickr, olx or ebay.in
2) No word of mouth, I would have to check it myself
3) Trust only good friends, whom you know, spread your requirement through word of mouth
4) Avoid dealers of used electronics (however honest they may appear to be)
5) Even if the deck has the slightest of defects/issues, back out, dont buy
6) Shortlist a few specific models and stick by them, don't window-shop
7) Be suspicious of low prices, the lower the price, the poorer the condition
8) No ebay.com, equipment with motors generally run poorly on 220v to 110v converters
9) You can't be too price consicious. You must be ready to spend. If you want a pristine deck purchased new (back in the day) by the single owner, he's going to charge you more because he paid more for it
10) Read up as much as possible on cassette decks in general and on the models you've shortlisted. Even single model has its unique characteristics, functionality and more importantly, the usual issues that can arise
11) listen, listen, listen, record, record, record (and playback of course). Many people make the mistake of not audiotioning. Also if you have an existing tape collection, you may want to play one of your favourite tapes on the deck so that you can make a comparison to what you've heard in the past. Another very important fact is that these days, audiophiles just say - who wants to record cassette tapes, I just want to play some of my old tapes - and they don't check the recording functionality. If your deck records perfectly (which is very critical for a cassette deck like the taking off and landing of an aircraft) then in all likely hood, it will play properly. Recording helps ascertain the condition of the R/P head (for 2-headers), condition of the erase head, bias calibration and most importantly, speed accuracy of the cassette deck. A cassette deck which is running slow or fast or has wow and flutter, may get away during play back but would be badly exposed during recording.
12) If not in good cosmetic condition, back out. A poor looking deck is usually a poorly handled and maintained deck.
Using this, I was able to get my Nakamichi DR3 in mint condition, from a single owner who purchased it new, and I am enjoying every moment of owning it. My NAK is the star of my rack indeed. If I have to sell off all my equipment, the NAK would be what I would sell at the very end and that too with a lot of difficulty.
Don't part with your NAK. Be patient, there are loads of NAK users who are very active on various internet hi-fi forums. Spare parts are also available but have to be sourced from abroad. Remember, if you have a NAK deck, its your prized possession by default because if you let it go, you are not likely to ever get one again. Also, if OEM spares are used, NAKs can be easily restored to their former glory. Their build quality is superb and with the finest components.
Keep consulting. From what you say about the spindle not turning (assume you are referring to the capstan which engages the pinch roller), I would suspect a broken belt. Imagine selling off your deck because it has a broken belt...
Just my 2 cents.