My advice to source from scratch for a full build - no. Been there, done that for 8275.
1) The cost of most small parts are cheap, but you have to add in postage cost from several sellers.
2) The cost of bigger parts, or rare item, or motor/LA/IR/etc. are more expensive. add in postage cost also.
3) Of course some can use substitute similiar colours, etc. but it won't feel good as a complete set.
4) If unlucky, you may get taxed from Malaysian Customs. Although I have never been ... yet.

If to modify an existing set, yes, then have to buy spare parts or buy whole box as spares. e.g. I bought 2nd 9398 to break it up as spare parts.

41999 - nice to have if you got the full box during launch at cheaper price than now. MOC it from 9398 - no. You might as well MOC other vehicles, check Jurgen, Sariel and other people's MOC on Technic. Reason, 41999 is using the same 9398 base, only the top cover is different. Mechanical wise is the same.

On your last sentence, that is the reason I build technics model-B first, before disassemble and build model-A to keep.

Just my 2 cents

BrickLink is your best bet. Some local sellers. Otherwise from Thai/SG/Taiwan/HK where postage is cheaper.
Last option is Europe/US.

Unit cost wise, generally is cheaper from Europe/US, compared to Asia.

To me, there is a big difference in playability between a bulldozer/front-loader (8275/42030) vs excavator (8043) vs crane (42009/42042) vs RC (9398/41999), even if it is completely modified.

My opinion on playability, lowest to highest:

Bulldozer/front-loader. It just bull-doze forward action only. RC with either chain or tyre. Not much playability.

Lego Crane - Most of the time only the hook lowers up and down. The weight liftage is a laugh. Crane will topple over easily for heavier weight 500gm+ or if the beam is fully extended. Some more advance will include motorized rotating super-structure. Even more advance/modified will include RC.

RC - Just RC only, moves/crawls faster than any other Lego models. I think plays better than crane.

Excavator - bucket can move. Arm can extend/retract. Super-structure can rotate. Wheel/Track can RC. Just refer to the above Jurgen's video. Full of playability and dexterity to control simultaneously.

Just my 2 cents I will still buy 42042

Depends how big is your table/floor. During my time, each set/pack comes in 2 pieces each (same type). Generally, I'll recommend in following ratio:
- 1 set curve
- 4 set cross
- 2 set straight
- 2 set T-junction

18 pieces 32x32 studs, if put in 6x3, that's about 1m x 3m.
If you slot in city sets or modular, the land area is even bigger.

You can cut down on those plates.
e.g. Curve is nice to have, but not really that useful.
Cross-roads can mod into T or straight

You can get building instructions/ideas here.

For more classic set, I can find my old set here
In those days, we need to use some imagination/logic to build a small vehicle with 1 single instruction page. A trailer/container took 4 pages only. Helicopter 2 pages
Nowadays, is like spoon-feeding.

LEGO® 9V Technic Motors compared characteristics

1) You need to by-pass the empty 3x AA cells. Or use own battery pack contraption.
2) The speed will be very slow.
3) If the ferris wheel needs high torque to start turning, you may need to give it a push.
4) Maintaining speed for long time may reduce the lifespan of the motor if the load is high due to low power.

Some cost effective to get RC is either:
1) buy the cheapest train set for the IR. One remote can actually control many IR.
2) buy 9398 (?) which used to be a good set to breakdown, and you get extra motors, etc.

You will also get a lot of parts to resell or use.

Not yet assemble

Ready ones:
8043 Jurgen's Ultimate
8110 motorized RC


Emerald Night 2k+ ? I bought it at about RM950 after conversion about 2 years ago.




For those not aware, some fixes to smoothen the gears or prevent it fall off while on track.

Not many active Technic owners here. It depends what model you want to mod or build from scratch ?
Also, if you have the end-model or guide, normally there is a list of parts you need.
Then perhaps forumers can give their advise/suggestion.

First thing first, Lego RC crawls at 1-2 feet per second, do not compare to those real RC toy cars.

Here's a starting point. Most of Technic mods need to do some own test build to find the parts needed first, as they don't have full instructions.
Then only see whether to buy a box or parts.
As in the video above, it's quite informative already on the major parts and locations (at least to me).