Paintguy is right. Here's my story. Too late to help now but maybe someone else will benefit. I gel stained my Pella fiberglass entry door 6.5 years ago and it looked great! Used Minwax gel stain and their spar varnish, as directed by Pella. However, it did not stand up to weather at all. It chipped and had to be varnished again every 6 months. This Spring it chipped all the way through to the fiberglass and pulled stain with it! Believe me, I did everything properly. My door faces south east and has no roof. But a neighbor whose fiberglass door faces north had the same trouble - he used the same products as I. My neighbor got disgusted and stripped his down to fiberglass and restained it with Sikkens products 3 years ago. No trouble since. Sikkens makes a Door and Window stain that does not require varnish over top. Fortunately for me, just before I started stripping about 36 layers of varnish off my door, it delaminated! Pella brought me a new unfinished slab (under warranty) and I'm using Sikkens to stain it. I've been told the varnish is the trouble. It's like putting a sheet of glass against the door, trapping the heat. Plus it isn't flexible enough to expand and contract. If I put up a storm door to protect it, the problem would've been worse due to trapped heat. Sikkens is specifically made for fiberglass doors. I don't have any vested interests in the company but thought I'd try to help the next guy... Here is a link that might be useful: Sikkens Door and Window stain

Should be fine when primed and painted.

I own a collection of all wood Marvins - no clad for me either due to being a historic property. Most of mine are the VG fir exterior and interior upgrade, but I do have one pine unit that was my first order (learned my lesson). I also looked at the Lowen products closely and my neighbors house is done with them. Lowen are nice, but have less options then Marvin and a very very limited dealer network. If things go south with your Lowen dealer, there is likely not another one in your state. The basic Marvin pine units are a mixture of solid pine and finger-jointed pine - if you read their architectural specs you will see this in small print, but not in the consumer catalog. In my experience the exterior trim, sill, etc were all finger jointed pine. My pine unit was primed outside and clear stain grade inside so the inside faces are solid or at least thick veneers. The jamb is also solid and cover up what lies beneath. Marvin's VG fir exterior and interior are a huge up-charge as you've seen, but the wood is solid without veneers (except if you get interior screens etc). Here in the NW a moist climate - finger jointed pine will rot for sure. From my experience, moisture kills pine and kills the glue in finger joints even worse. I had to replace some out just the other day - 12 years old - and painted/caulked every 2 years. Also, not entirely related, but since this is a historic property if you must match the existing brickmould/casing and Marvin doesn't have that profile you need without going to their Signature level, you can always get the windows without brickmould/casing (jambs only) and have your carpenters trim out the window with casing you can get reproduced locally at a nicer lumber yard. There is usually a ~$100-150 charge to replicate a knife for a trim profile, but you'll be ahead of signature costs and can have enough made to fix up any other windows with damage. Go fir here too. Finally, be careful if you are getting very large operable windows that are at or near the limits of their sizing chart - suggest mulling together two or more smaller units to create a similar total dimension.

Pricing wise, the wooden interior doors are more than double. The front door on the house also costs a good chunk more to get a stainable fiberglass material. If I have wood interior doors, I would want at least the inside portion of the front door to be wood, too. However, it is more of an oak and limited to gel stain kits, while my trim will definitely be pine. I feel like I may have too much wood in the house after wooden birch cabinets, stained pine base, stained pine window jambs, stained pine window casing, and stained pine door casing. Wooden doors would flow with the wood trim, but they would be big chunks of stained wood. Looking around online I see a lot of painted trim and stained wooden doors but NOT near as much stained trim and painted doors. However, I have wondered if I had doors painted a darker color, then maybe the wood stained trim would not look so broken up from the doors. Hoping to get a second opinion on this idea and if anyone has some photos of what I am talking about that'd be good to look over.