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With the rumors of an impending 11th Edition and a new Codex, is it best for me to wait a couple months to see what changes before heavily investing in an army?

Or should I just build what I want and hope that it's still usable in the near future? (While assuming things like Detachments will change or be removed).

It depends on what you intend to use the army for.

If you intend it for tournament play and meta chasing then you should wait a bit until 11th and the new meta is discovered.

If you want to use it more for fun and casual games then you can start right away and just get what ever you want. The thing about Space Marines is that they are the generalist army of the game and because they are the most popular pretty much everything (the older stuff, fortifications and flyers notwithstanding) is viable and can be used even at their worst.

As long as you magnetize everything, you can always change loadouts for every edition going forward

Historically, it's always been a good time to build space marines, and got new models in every launch/starter box. Beginning in 8th, with the shift to primaris, Marines began getting new units, not just new models, and with it new ways to play. there was a slow loss of firstborn units. Then, with 10th, Space Marines lost a large swathe of the remaining first born, keeping only Tacticals, Devestators, Vanguard Vets, and the Rhino/Razorback.

The smart money is that those final firstborn holdouts will be gone, possibly to be replaced either directly (like sternguard or terminators) or very closely (like assault intercessors with jump packs replacing assualt marines). Further, Stormcast in AOS had a surprise cull of relatively recent models. Marines are even more bloated, so I'd guess some stuff might get the ax.

What I would say is start with stuff you know isn't going anywhere, and build one of each unit. Stick with releases from the last two editions. Paint what you like, since there's no way of knowing what will be good.

IMO its almost always a good time to build an army, SM or otherwise.

But I play games in the here & now and am confident that I can make whatever I have work no matter what changes come along.

If i wanted to build a new army & the next edition was 6 months away (& it is, so mark your calender)?
I'd build & play the army.

Depends how niche your choices are and how bothered you will be if they are bad/unplayable.

If for example you went all/only terminators it's a potential risk.

It's also possible that Intercessors say are due a reprint. Would that annoy you if you got some now?

I'd say don't heavily invest in an army. Pick up a squad you like, make it, paint it. Then do another one, and another one.

By the time the new edition rolls around you won't have invested that heavily but you'll have a little start and can pick up the starter box and add to it.

If you're worried about stuff going obsolete just pick more recent kits and you'll probably be fine.

Right now I have the contents of the Dawn of War: Onslaught box

3 × Eradicators, 3 × Bladeguard Veterans, 1× Bladeguard Ancient, 1 × Judiciar, 1 × Chaplain.

(Still haven't decided on what chapter to paint them up as)

But I'm wary of spending money on units I won't be able to use in the foreseeable future.

I was burned badly by the Great Stormcast Eternal Purge in AoS.

 Lathe Biosas wrote:
Right now I have the contents of the Dawn of War: Onslaught box

3 × Eradicators, 3 × Bladeguard Veterans, 1× Bladeguard Ancient, 1 × Judiciar, 1 × Chaplain.

(Still haven't decided on what chapter to paint them up as)

But I'm wary of spending money on units I won't be able to use in the foreseeable future.

I was burned badly by the Great Stormcast Eternal Purge in AoS.

The only reason you were burned is because you choose not to use Legends.

ccs wrote:

 Lathe Biosas wrote:
Right now I have the contents of the Dawn of War: Onslaught box

3 × Eradicators, 3 × Bladeguard Veterans, 1× Bladeguard Ancient, 1 × Judiciar, 1 × Chaplain.

(Still haven't decided on what chapter to paint them up as)

But I'm wary of spending money on units I won't be able to use in the foreseeable future.

I was burned badly by the Great Stormcast Eternal Purge in AoS.

The only reason you were burned is because you choose not to use Legends.

Sadly, it isn't up to me.

I have a Warhammer store and an Unfriendly Local Game Store. I either leave the Legends rules at home or not play.

Those are my options.

 Lathe Biosas wrote:

ccs wrote:

 Lathe Biosas wrote:
Right now I have the contents of the Dawn of War: Onslaught box

3 × Eradicators, 3 × Bladeguard Veterans, 1× Bladeguard Ancient, 1 × Judiciar, 1 × Chaplain.

(Still haven't decided on what chapter to paint them up as)

But I'm wary of spending money on units I won't be able to use in the foreseeable future.

I was burned badly by the Great Stormcast Eternal Purge in AoS.

The only reason you were burned is because you choose not to use Legends.

Sadly, it isn't up to me.

I have a Warhammer store and an Unfriendly Local Game Store. I either leave the Legends rules at home or not play.

Those are my options.

Sounds like you need to make the Unfriendly Local Game Store friendlier.
Offer to run the next Crusade/Path to Glory. And since your the organizer, authorize Legends.