1. He spent much money on clothes.

Question

2. He spent too much money in buying books.
3. He spends lots of money traveling to many countries.
4. He spends plenty of time watching TV.
5. He spent a lot of time in buying items in the department store.

Are all grammatical? Do we have to use 'in' before the ing form? Do we have to drop 'in' before the ing form?

Answer 1

#1 would be better if you used "a lot" or "a great amount" instead of "much."

The rest are fine except for the use of "in" -- you don't need it in front of the -ing form.